U.S. patent number 9,726,234 [Application Number 14/391,924] was granted by the patent office on 2017-08-08 for clutch assembly.
This patent grant is currently assigned to Litens Automotive Partnership. The grantee listed for this patent is LITENS AUTOMOTIVE PARTNERSHIP. Invention is credited to Andrew M. Boyes, Jason R. Desouza-Coelho, Zbyslaw Staniewicz, Bashar Yazigi.
United States Patent |
9,726,234 |
Boyes , et al. |
August 8, 2017 |
Clutch assembly
Abstract
In an aspect, a clutch assembly is provided that uses an
electromagnet to generate a magnetic circuit which drives an
armature to a position whereat it engages the clutch to operatively
connect a rotating member to a stationary member. In embodiments
wherein the armature initially rotates with the rotating member,
the magnetic circuit passes through the stationary member. In
embodiments wherein the armature is initially stationary, the
magnetic circuit extends through the rotating member.
Inventors: |
Boyes; Andrew M. (Aurora,
CA), Desouza-Coelho; Jason R. (Markham,
CA), Staniewicz; Zbyslaw (Mississauga, CA),
Yazigi; Bashar (Woodbridge, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
LITENS AUTOMOTIVE PARTNERSHIP |
Woodbridge |
N/A |
CA |
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Assignee: |
Litens Automotive Partnership
(Woodbridge, Ontario, CA)
|
Family
ID: |
49326972 |
Appl.
No.: |
14/391,924 |
Filed: |
April 10, 2013 |
PCT
Filed: |
April 10, 2013 |
PCT No.: |
PCT/CA2013/000345 |
371(c)(1),(2),(4) Date: |
October 10, 2014 |
PCT
Pub. No.: |
WO2013/152430 |
PCT
Pub. Date: |
October 17, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20150041277 A1 |
Feb 12, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61622501 |
Apr 10, 2012 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B60K
25/02 (20130101); F16D 13/10 (20130101); F16D
27/105 (20130101); F16D 13/12 (20130101); F16D
13/76 (20130101); F16D 27/14 (20130101) |
Current International
Class: |
F16D
27/105 (20060101); F16D 27/14 (20060101); F16D
13/76 (20060101); F16D 13/12 (20060101); B60K
25/02 (20060101); F16D 13/10 (20060101) |
References Cited
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Other References
Warner Electric-Wrap Spring Clutch Catalog (p. 1310), Jul. 1, 2011,
Warner Electric. cited by applicant .
Texas Instruments--InstaSPIN BLDC Motor Control Lab, Nov. 1, 2011,
Texas Instruments. cited by applicant .
International Search Report and Written Opinion for
PCT/CA2013/000345, Jul. 19, 2013, ISA. cited by applicant .
International Preliminary Report on Patentability for
PCT/CA2013/000345, Oct. 23, 2014, ISA. cited by applicant .
Borg Warner Transmission Group Clutch Bearing Designs, Unknown,
Borg Warner Inc. cited by applicant .
Applimotion UTS--12VDC Frameless Ring Motors, 2014, Applimotion
Inc. cited by applicant .
CN201280049474.4 Office Action and search report dated Sep. 24,
2015. cited by applicant .
CN201280049474.4 English translation of search report dated Sep.
24, 2015. cited by applicant .
CN201280049474.4 English translation of Office Action dated Sep.
24, 2015. cited by applicant .
Extended European Search Report for EP12838370 dated Jun. 10, 2016.
cited by applicant .
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2013. cited by applicant.
|
Primary Examiner: Siconolfi; Robert A
Assistant Examiner: Dodd; Ryan
Attorney, Agent or Firm: Millman IP Inc.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent
Applications No. 61/622,501, filed Apr. 10, 2012, the contents of
which are incorporated by reference as if fully set forth in detail
herein.
Claims
The invention claimed is:
1. A clutch assembly, comprising: a first, input, clutch member
that is rotatable about an axis; a second, output, clutch member
that is rotatable about the axis; a wrap spring having a first end,
a second end and a plurality of helical coils between the first end
and the second end, wherein one of the first and second clutch
members is rotationally operatively connected with the first end of
the wrap spring; an armature that is rotationally operatively
connected to the second end of the wrap spring; an actuator,
wherein the actuator is rotationally operatively connected to the
second end of the wrap spring and the armature is rotationally
operatively connected to the actuator, but is slidable axially
relative to the actuator; and an electromagnetic unit including an
electromagnet, wherein energization of the electromagnet generates
a magnetic flux that flows through the other of the first and
second clutch members rotationally operatively connected with the
wrap spring first end, through the armature and back into the
electromagnetic unit, wherein, when the first clutch member is
rotating and the second clutch member is stationary, the magnetic
flux draws the armature axially into engagement with the other of
the first and second clutch members rotationally operatively
connected with the wrap spring first end with sufficient force to
frictionally cause rotation of the armature and the second end of
the wrap spring relative to the first end of the wrap spring so as
to radially expand the wrap spring into engagement with the other
of the first and second clutch members rotationally operatively
connected with the wrap spring first end thereby operatively
connecting the first clutch member to the second clutch member;
wherein the armature has an axial length and a radial thickness,
the axial length being greater than the radial thickness and the
radial thickness being selected to permit the armature to become
magnetically saturated at a selected voltage and at a selected
temperature such that the magnetic force exerted between the other
of the first and second clutch members and the armature is
substantially constant over a selected range of operating
temperatures and voltages.
2. A clutch assembly as claimed in claim 1, wherein the actuator is
made from a substantially non-magnetizable material.
3. A clutch assembly as claimed in claim 1, wherein the material of
the actuator has a lower density than the material of the
armature.
4. A clutch assembly as claimed in claim 1, wherein the
electromagnetic unit is mountable to a substantially
non-magnetizable engine member.
5. A clutch assembly as claimed in claim 1, wherein the armature
has a friction engagement surface that is frictionally engageable
with the other of the first and second clutch members rotationally
operatively connected with the wrap spring first end, and through
which the magnetic flux travels from the other of the first and
second clutch members rotationally operatively connected with the
wrap spring first end to the armature to draw the armature into
engagement with the other of the first and second clutch members
rotationally operatively connected with the wrap spring first end,
wherein the friction engagement surface has a surface area that is
selected to limit the frictional force between the other of the
first and second clutch members rotationally operatively connected
with the wrap spring first end and the armature.
6. A clutch assembly as claimed in claim 5, wherein the surface
area of the friction engagement surface is selected to provide a
selected area of contact between the armature and the other of the
first and second clutch members, and wherein the armature has a
cross-sectional surface area at an axial distance from the friction
engagement surface that is smaller than the surface area of the
friction engagement surface and is selected to control the
saturation limit for magnetization of the armature during
energization of the electromagnet.
7. A clutch assembly as claimed in claim 1, wherein the
electromagnetic unit has an armature flux transfer surface that is
spaced radially from the armature and that has a selected amount of
axial overlap with the armature, and wherein the electromagnetic
unit has a second clutch member flux transfer surface that is
spaced radially from the second clutch member and that has a
selected amount of axial overlap with the second clutch member.
8. A clutch assembly as claimed in claim 1, wherein, when the
electromagnet is not energized, the wrap spring is wrapped around a
portion of the first clutch member that has a radius that is larger
than a free state radius of the wrap spring, so as to generate a
selected amount of preload into the wrap spring.
9. A clutch assembly as claimed in claim 1, wherein, when the first
clutch member is rotating and the second clutch member is
stationary, the magnetic flux draws the armature axially into
engagement with the second clutch member with sufficient force to
frictionally retard the armature and the second end of the wrap
spring relative to the first end of the wrap spring so as to
radially expand the wrap spring into engagement with the second
clutch member thereby operatively connecting the first clutch
member to the second clutch member.
10. A clutch assembly as claimed in claim 1, wherein, when the
first clutch member is rotating and the second clutch member is
stationary, the magnetic flux draws the armature axially into
engagement with the first clutch member with sufficient force to
frictionally drive the armature and the second end of the wrap
spring rotationally about the axis relative to the first end of the
wrap spring so as to radially expand the wrap spring into
engagement with the first clutch member thereby operatively
connecting the first clutch member to the second clutch member.
11. A clutch assembly as claimed in claim 1, wherein at least one
of the first and second clutch members includes a resilient
isolation member that has a first end operatively connected to an
upstream member and a second end connected to a downstream member,
wherein the isolation member transfers torque between the upstream
and downstream members.
12. A clutch assembly as claimed in claim 1, further comprising a
return spring that urges the armature axially away from the other
of the first and second clutch members.
13. A clutch assembly as claimed in claim 1, wherein the armature
is configured to be substantially saturated at a high end of
operating temperature range and at low end of operating voltage
range.
Description
FIELD
The present disclosure relates to drive systems for transferring
power from an output shaft of an engine to an input shaft of a
load, and more particularly to such loads as a supercharger, an
alternator, a cooling fan, a power steering pump, an
air-conditioning compressor, a vacuum pump, an air compressor, a
hydraulic motor, a power take off, a secondary electrical generator
or any other suitable kind of load.
BACKGROUND
Clutches are useful devices for controlling the operative
connection between a drive element, such as an engine crankshaft in
a vehicle, with a driven element, such as an accessory in the
vehicle, such as, for example, a supercharger, an alternator or any
other suitable accessory. However, many clutches currently suffer
from a number of problems. Some clutches require a significant
amount of power to operate unfortunately, and therefore require
electrical cable that is capable of carrying high currents, as well
as relays and the like, which add to the cost associated with such
clutches, aside from their large power draw.
Some clutches which have a lower power draw still require many
components even though they do not require relays, high-current
electrical cable and the like.
Some clutches are very sensitive to the gaps between certain
components and are consequently very difficult to install,
requiring careful shimming of components during their installation
to ensure that gaps between components are maintained.
It would be beneficial to provide a clutch that at least partially
addresses one or more of these issues.
SUMMARY
In an aspect, a clutch assembly is provided that uses an
electromagnet to generate a magnetic circuit which drives an
armature to a position whereat it engages the clutch to operatively
connect a rotating member to a stationary member. In embodiments
wherein the armature initially rotates with the rotating member,
the magnetic circuit passes through the stationary member. In
embodiments wherein the armature is initially stationary, the
magnetic circuit extends through the rotating member.
In an embodiment, the clutch assembly includes a wrap spring having
a first end, a second end and a plurality of helical coils between
the first end and the second end. One of the first and second
clutch members is rotationally operatively connected with the first
end of the wrap spring. An armature is provided and is rotationally
operatively connected to the second end of the wrap spring. An
electromagnetic unit is provided and includes an electromagnet.
Energization of the electromagnet generates a magnetic flux that
flows through one of the first and second clutch members, through
the armature and back into the electromagnetic unit. When the first
clutch member is rotating and the second clutch member is
stationary, the magnetic flux draws the armature axially into
engagement with the other of the first and second clutch members
with sufficient force to frictionally cause rotation of the
armature and the second end of the wrap spring relative to the
first end of the wrap spring so as to radially expand the wrap
spring into engagement with the other of the first and second
clutch members thereby operatively connecting the first clutch
member to the second clutch member.
In an embodiment, the clutch assembly includes a first clutch
member that is rotatable about an axis, a second clutch member that
is rotatable about the axis, a wrap spring, an armature and an
electromagnetic unit. The wrap spring has a first end, a second end
and a plurality of helical coils between the first end and the
second end. The first clutch member is rotationally operatively
connected with the first end of the wrap spring. The armature is
rotationally operatively connected to the second end of the wrap
spring. The first clutch member is rotationally operatively
connected to the armature. The electromagnetic unit includes an
electromagnet. Energization of the electromagnet generates a
magnetic flux that flows through the second clutch member, through
the armature and back into the electromagnetic unit. When the first
clutch member is rotating and the second clutch member is
stationary, the magnetic flux draws the armature axially into
engagement with the second clutch member with sufficient force to
frictionally retard the armature and the second end of the wrap
spring relative to the first end of the wrap spring so as to
radially expand the wrap spring into engagement with the second
clutch member thereby operatively connecting the first clutch
member to the second clutch member.
In another embodiment, the clutch assembly includes a first clutch
member that is rotatable about an axis, a second clutch member that
is rotatable about the axis, a wrap spring, an armature and an
electromagnetic unit. The wrap spring has a first end, a second end
and a plurality of helical coils between the first end and the
second end. The first end is rotationally operatively connected
with the second clutch member. The armature is rotationally
operatively connected to the second end of the wrap spring. The
electromagnetic unit includes an electromagnet. Energization of the
electromagnet generates a magnetic circuit that transports a
magnetic flux through the first clutch member, through the armature
and back into the electromagnetic unit. When the first clutch
member is rotating and the second clutch member is stationary, the
magnetic flux draws the armature axially into engagement with the
first clutch member with sufficient force to frictionally drive the
armature and the second end of the wrap spring rotationally about
the axis relative to the first end of the wrap spring so as to
radially expand the wrap spring into engagement with the first
clutch member thereby operatively connecting the first clutch
member to the second clutch member.
In another aspect, a clutch assembly is provided that includes a
first clutch member that is rotatable about an axis, a second
clutch member that is rotatable about the axis, a wrap spring and a
wrap spring engagement drive structure. The wrap spring has a first
end, a second end and a plurality of helical coils between the
first end and the second end. The first clutch member is
rotationally operatively connected with the first end of the wrap
spring. The wrap spring engagement drive structure is rotationally
operatively connected with the second end of the wrap spring.
Movement of the wrap spring engagement drive structure from a
disengaged position wherein the wrap spring is unengaged with the
second clutch portion to an engaged position wherein the wrap
spring engagement drive structure drives the second end of the wrap
spring rotationally relative to the first end of the wrap spring to
expand the wrap spring into engagement with the second clutch
member to operatively connect the first clutch member to the second
clutch member. When the wrap spring engagement drive structure is
in the disengagement position, the wrap spring is wrapped around a
wrap spring support surface that has a radius that is larger than a
free state radius of the wrap spring, so as to generate a selected
amount of preload into the wrap spring.
In another aspect, a clutch assembly is provided that includes a
first clutch member that is rotatable about an axis, a second
clutch member that is rotatable about the axis, a wrap spring and a
wrap spring engagement drive structure. The wrap spring has a first
end, a second end and a plurality of helical coils between the
first end and the second end. A wrap spring engagement drive
structure is provided that is rotationally operatively connected
with the second end of the wrap spring. Movement of the wrap spring
engagement drive structure from a disengaged position wherein the
wrap spring is unengaged with the second clutch portion to an
engaged position wherein the wrap spring engagement drive structure
drives the second end of the wrap spring rotationally relative to
the first end of the wrap spring to expand the wrap spring into
engagement with the second clutch member to operatively connect the
first clutch member to the second clutch member. When the wrap
spring engagement drive structure is in the disengagement position,
the wrap spring is wrapped around a wrap spring support surface
that has a radius that is larger than a free state radius of the
wrap spring, so as to generate a selected amount of preload into
the wrap spring.
In yet another aspect, a combination of a wrap spring and a carrier
is provided. The wrap spring has a first end and a second end and a
plurality of coils between the first and second ends. The carrier
includes a slot sized to receive the first end of the wrap spring,
wherein the carrier includes a drive wall at an end of the slot
which is abutted by a helical end face at the first end of the wrap
spring. The carrier has a torque transfer surface configured to
transfer a torque from the first end of the wrap spring into
another component. The torque transfer surface has a greater
surface area than the helical end face at the first end of the wrap
spring.
In some embodiments, the torque transfer surface is arcuate so as
to be pivotable within a complementary arcuate torque transfer
surface of a component that is engaged by the carrier.
In yet another aspect, a combination of a wrap spring and a carrier
is provided. The wrap spring has a first end and a second end and a
plurality of coils between the first and second ends. The carrier
includes a slot sized to receive the first end of the wrap spring.
The carrier includes a drive wall at an end of the slot which is
abutted by a helical end face at the first end of the wrap spring.
The carrier has a torque transfer surface configured to transfer a
torque from the first end of the wrap spring into another
component. The torque transfer surface is arcuate so as to be
pivotable within a complementary arcuate torque transfer surface of
a component that is engaged by the carrier.
In another aspect, a clutch assembly is provided that includes a
first clutch member that is rotatable about an axis, a second
clutch member that is rotatable about the axis, a wrap spring and a
carrier. The wrap spring has a first end, a second end and a
plurality of helical coils between the first end and the second
end. The carrier includes a slot sized to receive the first end of
the wrap spring. The carrier includes a drive wall at an end of the
slot which is abutted by a helical end face at the first end of the
wrap spring. The carrier has a torque transfer surface configured
to transfer a torque from the first end of the wrap spring into
another component. The torque transfer surface is arcuate so as to
be pivotable within a complementary arcuate torque transfer surface
associated with the first clutch member.
In some embodiments, the torque transfer surface on the carrier has
a surface area that is larger than a surface area of the helical
end face at the first end of the wrap spring.
In some embodiments, the wrap spring is urged to expand radially
and pivot the carrier in a first direction during torque transfer
from the first clutch member into the wrap spring through the
carrier. The carrier further includes a guide surface that is
engageable with a corresponding guide surface associated with the
first clutch member when torque is not transferred from the first
clutch member into the wrap spring, so as to urge pivoting of the
carrier in a second direction corresponding to radial contraction
of the wrap spring.
In another aspect, a clutch assembly is provided that uses an
electromagnet to generate a magnetic circuit which drives an
armature to a position whereat it engages the clutch to operatively
connect a rotating member to a stationary member. In embodiments
wherein the armature initially rotates with the rotating member,
the magnetic circuit passes through the stationary member. In
embodiments wherein the armature is initially stationary, the
magnetic circuit extends through the rotating member. The armature
has a size that is selected to permit the armature to become
magnetically saturated at a selected voltage and at a selected
temperature, so that the magnetic force exerted between the member
that has the magnetic circuit passing through it and the armature
is substantially constant over a selected range of temperatures and
voltages.
In yet another aspect, a clutch assembly is provided that uses an
electromagnet to generate a magnetic circuit which drives an
armature to a position whereat it engages the clutch to operatively
connect a rotating member to a stationary member. In embodiments
wherein the armature initially rotates with the rotating member,
the magnetic circuit passes through the stationary member. In
embodiments wherein the armature is initially stationary, the
magnetic circuit extends through the rotating member. The armature
has a friction engagement surface that is frictionally engageable
with the member that has the magnetic circuit passing through it,
and a flux transfer surface through which the magnetic flux travels
from the member that has the magnetic circuit passing through it to
the armature to draw the armature into engagement with the member
that has the magnetic circuit passing through it. The friction
engagement surface has a surface area that is selected to limit the
frictional force between the member that has the magnetic circuit
passing through it and the armature. The flux transfer surface is
selected to provide a selected area of contact between the armature
and the member that has the magnetic circuit passing through it.
The armature has a cross-sectional surface area at an axial
distance from the flux transfer surface that is smaller than the
surface area of the flux transfer surface and is selected to
control the saturation limit for magnetization of the armature
during energization of the electromagnet. In some embodiments, the
flux transfer surface is the same surface as the friction
engagement surface.
BRIEF DESCRIPTION OF THE DRAWINGS
The present disclosure will now be described, by way of example
only, with reference to the attached drawings, in which:
FIG. 1 is a side view of a clutch assembly in accordance with an
embodiment of the invention;
FIG. 2a is a perspective exploded view of the clutch assembly shown
in FIG. 1;
FIG. 2b is another perspective exploded view of the clutch assembly
shown in FIG. 1;
FIGS. 3a and 3b are sectional side views of the clutch assembly
shown in FIG. 1, in disengaged and engaged positions
respectively;
FIG. 3c shows a magnetic circuit formed when the clutch assembly
shown in FIG. 1 is in the engaged position;
FIGS. 4a, 4b and 4c are magnified perspective exploded views of
portions of the clutch assembly shown in FIG. 1;
FIG. 5 is a graph showing test results for the force to hold the
clutch assembly in accordance with an embodiment of the invention
in an engaged position;
FIGS. 6a and 6b are perspective exploded views of a clutch assembly
in accordance with another embodiment of the invention;
FIGS. 7a and 7b are sectional side views of the clutch assembly
shown in FIGS. 6a and 6b, in disengaged and engaged positions
respectively;
FIG. 8 is a sectional side view of a portion of an alternative
clutch assembly that is similar to the embodiment shown in FIGS. 6a
and 6b, that includes an optional structure that facilitates
installation of the clutch assembly to a driven accessory;
FIG. 8a are views of tools that are used for the installation of
the clutch assembly shown in FIGS. 6a and 6b to an accessory;
FIG. 9 is a sectional side view of a portion of an alternative
clutch assembly that is similar to the embodiment shown in FIGS. 6a
and 6b, but that includes an optional decoupler;
FIGS. 10a, 10b and 10c are sectional side views of alternative
shapes for an armature that can be used in the clutch assemblies
shown in FIGS. 1 and 6a and 6b;
FIGS. 11a and 11b are exploded perspective views of another
embodiment of a clutch assembly;
FIG. 11c is a sectional side view of the clutch assembly shown in
FIGS. 11a and 11b;
FIGS. 12a and 12b are graphs illustrating data relating to
components of the clutch assembly shown in FIGS. 11a and 11b that
can be achieved using a control system for the clutch assembly;
FIG. 13 is a schematic diagram of a portion of the control
system;
FIGS. 14a and 14b are diagrams illustrating a method of controlling
the clutch assembly shown in FIGS. 11a and 11b;
FIGS. 15a and 15b perspective views of a wrap spring and a carrier
for the clutch assembly shown in FIGS. 11a and 11b;
FIG. 16 is an exploded perspective view of some of the components
of the clutch assembly shown in FIGS. 11a and 11b;
FIG. 17 is a perspective view of the components shown in FIG.
16;
FIGS. 18a-18c illustrate operation of a carrier for the wrap
spring;
FIG. 19a is another sectional side view of the clutch assembly
shown in FIGS. 11a and 11b; and
FIG. 19b is a sectional elevation view of the clutch assembly taken
along section line 19b-19b in FIG. 19a.
DETAILED DESCRIPTION
Reference is made to FIG. 1, which shows an engine crankshaft 10 of
an internal combustion engine 12 (represented for convenience by
lines defining a rectangular volume). The crankshaft 10 is
rotatable about an axis A. A clutch assembly 14 (which may also be
referred to as a clutch 14) is mounted to the crankshaft 10 and is
operable to selectively connect the crankshaft 10 to a selected
accessory (not shown). The accessory may be any suitable accessory,
such as, for example, a supercharger, an alternator, a water pump,
a fan, an air conditioning compressor, a power steering pump, a
vacuum pump, an air compressor, a hydraulic motor, a power take off
or a secondary electrical generator.
Referring to FIGS. 2a and 2b, the clutch assembly 14 includes a
first clutch member 16 and a second clutch member 18 that are both
rotatable about the axis A, a wrap spring 20, an armature 22, an
actuator 24 and an electromagnetic unit 26. The clutch assembly 14
can be constructed from relatively few components, and is usable to
selectively connect the crankshaft 10 to the accessory using very
low power.
The clutch assembly 14 is movable from a disengaged position shown
in FIG. 3a to an engaged position shown in FIG. 3b by transmission
of a magnetic flux from the electromagnetic unit 26 through the
second clutch member 18, the armature 22 and back into the
electromagnetic unit 26. The clutch assembly 14 may be referred to
as being engaged when it in the engaged position and may be
referred to as being disengaged or unengaged when it is in the
disengaged position.
The first clutch member 16 is driven by the crankshaft 10 and in
the embodiment shown in FIG. 1 it mounts to the crankshaft 10. The
first clutch member 16 may be made from any suitable material, such
as a suitable steel.
The second clutch member 18 is driven by the first clutch member 16
when the clutch 14 is engaged (FIG. 3b), and may be idle when the
clutch 14 is disengaged (FIG. 3a). The second clutch member 18 may
be rotatably supported on the first clutch member 16 by means of
one or more bearing members 28. In the embodiment shown there is a
single bearing member 28 provided, which is a ball bearing, which
is held on the second clutch member 18 by means of a bearing
retainer 29 that is fixedly mounted to the second clutch member 18
(e.g. by way of a press-fit).
The second clutch member 18 may be configured to transfer power
from the crankshaft 10 to the accessory in any suitable way. For
example, in the embodiment shown in FIG. 1, the second clutch
member 18 is a pulley 32 that is configured to engage a belt (not
shown) that engages a pulley on the input shaft of the accessory.
In alternative embodiments however, the second clutch member 18 may
be some other suitable power transfer element such as a gear that
engages one or more gears that ultimately drive the accessory, a
sprocket that drives a chain or the like that ultimately drives the
accessory.
The second clutch member 18 may be made from material that has at
least a selected permeability so that it has at least a selected
capability to transfer a magnetic flux, such as a 1010 steel. It
will be noted that it is not important for the first clutch member
16 to be made from a material having a particularly high magnetic
permeability, or a particularly low magnetic permeability. The
magnetic permeability of the first clutch member 16 is not
important, at least in the embodiment shown in FIG. 1.
The first clutch member 16 may optionally have a means for driving
other accessories that are separate from the accessory. For
example, in an embodiment wherein the accessory is a supercharger,
the first clutch member 16 may be configured to drive accessories
such as an alternator, a water pump, an air conditioning
compressor, a power steering pump, a fan, a power steering pump, a
vacuum pump, an air compressor, a hydraulic motor, a power take off
or a secondary electrical generator, separately from the accessory
that is driven from the second clutch member 18. The means for
driving these other accessories may be provided by a second pulley
34 or some other suitable power transfer member that is mounted to
the first clutch member 16 separately from the second clutch member
18. In such embodiments, as shown in FIG. 1, the second clutch
member 18 may be referred to a first power transfer member for
driving one or more first accessories, and the second pulley 34 may
be referred to as a second power transfer member for driving one or
more second accessories.
This second pulley 34 may include a means for damping torsional
vibration that may be generated at the crankshaft 10 (a by-product
of the operation of many, if not all, internal combustion engines).
For example, the second pulley 34 may include a rubber damping
element 35 therein, between an inner portion 36 of the pulley 34
and an outer portion 38 of the pulley 34. In some embodiments, the
second pulley 34 may be replaced by a torsional vibration damping
disc that is not intended to drive any components, but is there
only to dampen torsional vibrations from the engine 12.
The wrap spring 20 is movable between a disengaged position shown
in FIG. 3a and an engaged position shown in FIG. 3b. In the
disengaged position the wrap spring 20 is unengaged with the second
clutch member 18 and first clutch member 16 is operatively
disconnected with the second clutch member 18 (i.e the clutch 14 is
disengaged). In the engaged position the wrap spring 20 is radially
expanded into engagement with a radially inner surface shown at 39
of the second clutch member 18 (i.e. the clutch 14 is engaged).
The wrap spring 20 has a first end 40 (shown best in FIG. 4a), a
second end 42 (shown best in FIG. 4b) and a plurality of helical
coils 44 between the first end 40 and the second end 42. The first
clutch member 16 is engaged with the first end 40 of the wrap
spring 20 via engagement of a lug 52 (FIG. 4c) with the helical end
face of the first end of the wrap spring 20. The first end 40 of
the wrap spring 20 may be held in a groove 46 (FIG. 4b) in a
carrier shown at 48.
The carrier 48 in general assists in maintaining a predetermined
shape to the wrap spring 20, and assists the wrap spring 20 in
resisting undesired deformation particularly during periods in
which the wrap spring 20 is transferring high torque from the first
clutch member 16 to the second clutch member 18.
The carrier 48 may be made any suitable material, such as a plastic
material, or alternatively a metallic material.
Referring to FIG. 3a, the carrier 48 is held in a carrier seat 50
in the first clutch member 16. The carrier 48 is driven
rotationally about the axis A by the first clutch member 16 by
means of engagement between a plurality of lugs 52 (shown in FIG.
4c) on the first clutch member 16 and a plurality of lug slots 54
(FIG. 4a) on the carrier 48. While two lugs 52 and two lug slots 54
are shown, in some embodiments a single lug 52 and a single lug
slot 54 could be provided, or three or more lugs 52 and lug slots
54 could be provided. Instead of providing lugs 52 on the first
clutch member 16 and lug slots 54 on the carrier 48, the lugs 52
could be on the carrier 48 and the lug slots 54 could be on the
first clutch member 16.
The carrier 48 further includes a retainer engagement surface 56
(FIG. 3a). A retainer 58 that is press-fit onto the first clutch
member 16 engages the retainer engagement surface 56 and holds the
carrier 48 in place against the carrier seat 50.
Referring to FIG. 4b, the carrier 48 further includes a wrap spring
seat 51 on which the wrap spring 20 rests. The wrap spring seat 51
terminates in the groove 46. The groove 46 ends at one of the lug
slots 54. When lugs 52 (FIG. 4c) are positioned in the lug slots 54
the first end 40 of the wrap spring 20 directly engages one of the
lugs 52. As a result, the first clutch member 16 does not drive the
wrap spring 20 through the carrier 48, but instead drives the first
end 40 of the wrap spring 20 directly. This is advantageous in that
the helical end face 20a of the first end 40 of the wrap spring 20
directly abuts (and is driven by) a metallic surface (i.e. the lug
52) instead of abutting material from the carrier 48, which may be
softer than the material of the first clutch member 16 and which
could be deformed by the helical end face 20a if it were present
between the end face 20a and the lug 52 during periods of high
torque transfer.
The carrier 48 further includes a first portion 59 of a wrap spring
support surface 60 that extends axially. The first portion 59 of
the surface 60 supports a portion of the radially inner surface
(shown at 61) of the wrap spring 20 (FIG. 3a).
The actuator 24 may be made from a material that can slide against
the material of the first clutch member 16 and is mounted on the
first clutch member 16 so as to be rotational slidable thereon. For
example, the actuator 24 may be made from a polymeric material that
can slip relative to the material of the first clutch member 16
(which may be metallic).
An actuator retainer 63 is fixedly mounted to the first clutch
member 16 to prevent axial movement beyond a selected axial
distance relative to the second clutch member 18. In an embodiment,
the actuator 24 may be made from a material that is at least in
some embodiments non-magnetic, such as Nylon 4-6 that is modified
to include Teflon.TM., or alternatively aluminum (which may be pure
aluminum or an aluminum alloy). The actuator 24 supports the
armature 22 in such a way that the armature 22 is movable axially
thereon but such that the armature 22 is rotationally operatively
connected to the actuator 24. For greater certainty, when a first
object is `rotationally operatively connected` to or with a second
object, this means that the first object is capable of causing
rotation in the second object, without limitation on whether or not
the first object is capable of causing axial movement in the second
object. Depending on how they are connected, the second object may
also be capable of causing rotation in the first object;
configurations where this is possible will be readily apparent from
the description and figures. The actuator 24 need not be made from
a non-magnetic material. In some embodiments it may be made from a
material that has less than a selected permeability. In other
embodiments it may have a relatively high permeability while being
separated from the electromagnetic unit housing 70 by a suitable
insulative air gap or by some magnetically insulative material.
As shown in FIG. 2b, this rotational operative connection may be
achieved by providing one or more lugs 62 on the actuator 24 and
one or more lug slots 64 on the armature 22. While the lugs 62 and
lug slots 64 permit the actuator 24 and the armature 22 to drive
each other rotationally, they permit the armature 22 to slide
axially between a first position shown in FIG. 3a, and a second
position shown in FIG. 3b. The first and second positions of the
armature 22 are described in further detail, further below.
Referring to FIGS. 3a and 3b and 2a, the actuator 24 further
includes a drive slot 66 that receives the second end 42 of the
wrap spring 20, thereby fixing the second end 42 of the wrap spring
to the actuator 24 and the armature 22. Thus it can be said that
there is an operative connection between the armature 22 and the
second end 42 of the wrap spring 20. It can be seen that the first
clutch member 16 is operatively connected to the armature for
rotation about the axis A, as a result of the operative connection
between the first clutch member 16 and the first end 40 of the wrap
spring 20 and the operative connection between the second end 42 of
the wrap spring 20 and the actuator 24 and therefore the armature
22.
It will be noted that the actuator 24 has thereon another portion
67 (FIG. 3a) of the wrap spring support surface 60, and which
supports another portion of the radially inner surface 61 of the
wrap spring 20 (FIG. 3a). Together the first and second portions 59
and 67 may make up some or all of the wrap spring support surface
60. The wrap spring support surface 60 has a selected radius that
is larger than a free state radius of the wrap spring 20, so as to
generate a selected amount of preload into the wrap spring 20 when
the wrap spring is supported thereon. In other words, if the wrap
spring 20 were permitted to, it would radially contract to a free
state having a radius that is smaller than the radius of the wrap
spring support surface 60. As a result, the wrap spring 20 is under
some tension (i.e. it is preloaded by some amount) even when it
rests on the wrap spring support surface 60. This preload causes
the wrap spring 20 to engage the support surface 60 with a certain
amount of force.
During use, when the first clutch member 16 rotates and the clutch
14 is disengaged centrifugal forces act on the wrap spring 20 from
the speed of rotation itself and urge it to radially expand.
Additionally, during use, the engine 12 (FIG. 1) may undergo
relatively strong accelerations (i.e. ramp-ups in engine speed)
during, for example, aggressive driving maneuvers or during
transmission downshifts, or even resulting from torsional vibration
from the engine 12. These accelerations may momentarily urge the
first end 40 of the wrap spring 20 away from the second end 42 in a
selected circumferential direction that urges the wrap spring 20 to
radially expand. If the wrap spring 20 had no preload in it,
substantially any force urging it to radially expand would result
in at least some amount of radial expansion of the wrap spring 20
away from the support surface 60. This can result in noise when the
wrap spring 20 re-contacts the support surface 60 after the force
causing it to expand is removed or reduced. Also, if the force was
sufficiently strong, the wrap spring 20 could expand by a
sufficient amount to momentarily engage the inner surface 39 of the
second clutch member 18, thereby momentarily operatively connecting
the first and second clutch members 16 and 18. Depending on what is
driven by the second clutch member, this could result in a variety
of different problems. For example, if the accessory that is driven
by the second clutch member 18 is a supercharger, this could result
in additional air being transported into the combustion chambers of
the engine 12 when it is not expected by the Engine Control Unit
(ECU--not shown), resulting in turn in an unexpected change in the
stoichiometry of the air/fuel mixture in the combustion chambers.
This could lead to improper fuel combustion or other problems, and
could ultimately result in a fault being generated by the ECU when
it senses some unexpected change in engine performance brought on
by the inadvertent momentary operation of the supercharger. Aside
from noise and the potential for unintended operation of a driven
accessory, the wrap spring 20 could incur repeated expansion and
contraction if it was subject to vibration and was not preloaded.
This could result in wear, fatigue and ultimately a reduced
operating life for the wrap spring 20. By providing the
aforementioned preload in the wrap spring 20, the preload overcomes
these forces at least to some extent so as to provide the wrap
spring 20 with a selected amount of resistance to expand away from
the wrap spring support surface 60. As a result, problems with
noise and with inadvertent operation of the driven accessory may be
reduced or eliminated. The benefits of providing the preload in the
wrap spring 20 described above may be applicable to any structure
wherein the wrap spring 20 rotates with a first clutch member and
is selectively controllable to expand into engagement with a second
clutch member to operatively connect the first and second clutch
members, wherein a wrap spring engagement drive structure that
includes the electromagnetic unit and an armature is used. The
benefits of providing the preload in the wrap spring 20 may also be
applicable when any other suitable kind of wrap spring engagement
drive structure is used.
In the embodiment shown in FIGS. 3a and 3b, the wrap spring
engagement drive structure includes the armature 22, the actuator
24, the electromagnetic unit 26, and the second clutch member 18
itself.
Instead of providing a wrap spring support surface 60 that has a
larger radius than the free state radius of the wrap spring 20, in
an alternative embodiment the wrap spring 20 may be permitted to
contract all the way to its free state radius and it may have a
relatively larger radial spacing in that state from the inner
surface 39 of the second clutch member 18. By providing a large
radial spacing, even if the wrap spring expands under centrifugal
forces or engine accelerations, it will be unlikely to engage the
inner surface 39 of the second clutch member 18.
The armature 22 is preferably made from a material that has at
least a selected magnetic permeability but that also reaches
magnetic saturation under selected conditions, which are described
further below. The material of the actuator 22, however, may be
selected to have a relatively low magnetic permeability. This
inhibits magnetic flux from being transferred through the actuator
and into the electromagnetic unit 26.
In some embodiments, the face on the armature 22 that engages the
second clutch member 18, which may be referred to as the friction
engagement surface 82, may have a relatively high coefficient of
friction and may be largely responsible for generating a strong
friction force with the second clutch member 18. In some
embodiments, the friction engagement surface 82 may have a similar
coefficient of friction to the corresponding surface on the second
clutch member 18. In some embodiments it may be the corresponding
surface on the second clutch member 18 that has the relatively high
coefficient of friction.
With reference to FIG. 3a, in some embodiments the friction
engagement surface 82 is closer to the corresponding flux transfer
surface on the second clutch member 18 (shown at 80) than the
nearby flux transfer surface shown at 68 on the electromagnet
housing 70 is to the surface 80. This relative proximity of the
friction engagement surface 82 to the second clutch member 18
causes the magnetic flux to preferentially pass into the armature
22. It will be noted however, that even if some flux were
transferred from the second clutch member 18 directly into the
electromagnet housing 70, there would be a sufficient magnetic
force on the armature 22 to draw the armature 22 into engagement
with the second clutch member 18 and as the armature 22 began to
move towards the second clutch member 18, the flux lines would
begin to shift to preferentially pass into the armature 22 from the
second clutch member 18. It will be noted that this may occur even
in embodiments wherein the friction engagement surface 82 on the
armature 22 is positioned at the same distance from the mutually
facing surface of the second clutch member 18 as the nearby surface
on the electromagnetic unit 26, and even in some embodiments
wherein the friction engagement surface 82 on the armature 22 is
positioned a bit farther from the mutually facing surface of the
second clutch member 26 than the nearby surface on the
electromagnetic unit 26.
The electromagnetic unit 26 generates a magnetic flux that flows
through the second clutch member 18, the armature 22 and back into
the electromagnetic unit 26. The magnetic flux path (i.e. the
magnetic circuit) is generally illustrated by arrows 500 shown in
FIG. 3c). The electromagnetic unit 26 includes an electromagnet 69.
Energization of the electromagnet 69 generates the magnetic flux.
The electromagnetic unit 26 further includes an electromagnetic
unit housing 70 that holds the electromagnet 69. The
electromagnetic unit housing 70 connects to a clutch housing 71
that is configured to mount to a stationary member 72, which may
be, for example, the engine block or the engine cover. In a
preferred embodiment the engine block or whatever the stationary
member is, is made from a non-magnetizable material, such as, for
example, a type of aluminum (i.e. pure aluminum or an aluminum
alloy).
When the first clutch member 16 is rotating and the second clutch
member 18 is stationary the wrap spring 20, the actuator 24 and the
armature 22 rotate with the first clutch member 16. When it is
desired to engage the clutch assembly 14 (i.e. to bring the clutch
to the engaged position so as to operatively connect the first
clutch member 16 to the second clutch member 18), the
electromagnetic unit 26 is energized, generating a magnetic flux in
the second clutch member 18. This magnetic flux draws the armature
22 axially into engagement with the second clutch member 18 with
sufficient force to frictionally retard the armature 22 and the
second end 42 of the wrap spring 20 relative to the first end 40 of
the wrap spring 20. This movement of the second end 42 of the wrap
spring 20 causes the wrap spring 20 to radially expand into
engagement with the wrap spring engagement surface 39 on the second
clutch member 18 thereby operatively connecting the first clutch
member 16 with the second clutch member 18.
When the electromagnetic unit 26 is deenergized, there is no longer
a magnetic flux in the second clutch member 18, or there may remain
a small, residual magnetic flux in the second clutch member 18. As
a result, the force of engagement between the armature 22 and the
second clutch member 18 is greatly reduced, possibly to zero if
there is no longer any residual magnetic flux in the second clutch
member 18. As a result, the bias of the wrap spring 20 that urges
the wrap spring 20 towards its free state will overcome whatever
frictional force there may be between the armature 22 and the
second clutch member 18, and will thus cause the wrap spring 20 to
contract, and thus to retract from the inner surface 39 of the
second clutch member 18, thereby operatively disconnecting the
first clutch member 16 from the second clutch member 18. The clutch
14 may thus be referred to as being `normally disengaged`. This
provides a failsafe feature so that the clutch 14 does not drive
the pulley 18 (and the accessory or accessories driven by the
pulley 18) in situations where the clutch 14 has failed and driving
of the pulley 18 is not desired or is dangerous.
In the clutch assembly 14 it is generally desirable for the
magnetic force exerted between the second clutch member 18 and the
armature 22 to be relatively constant in every production unit and
under varying conditions, so that any tolerances in the properties
or dimensions of the components in each unit of the assembly 14 and
any variability in the operating conditions for a given unit do not
significantly affect this force. To that end, as shown in FIG. 3a
there is a selected, relatively large amount of axial overlap
between the second clutch member 18 and the electromagnetic unit 26
at their mutually facing flux transfer surfaces shown at 76 and 78
respectively. Furthermore, there is a selected, relatively large
amount of axial overlap between the electromagnetic unit 26 and the
armature 22 at their mutually facing flux transfer surfaces shown
at 81 and 83 (FIG. 3a) respectively, both when the armature is in a
disengaged position (i.e. it does not engage the second clutch
member 18) and when it is in an engaged position). These axial
overlaps are selected to be relatively large in order to ensure
that there is a relatively large axial overlap between the
aforementioned surfaces even when the clutch assembly 14 is
manufactured at the extremes of its dimensional tolerances. In this
way, the flux transfer between the second clutch member 18 and the
relatively thin friction engagement surface 82 of the armature is
where the flux transfer is the most restricted. This, in turn,
ensures that the configuration of the armature 22 is what controls
the magnitude of the magnetic force that holds the armature 22 in
engagement with the second clutch member 18 when the
electromagnetic unit 26 is energized. By contrast, if there were a
restriction at some other point in the magnetic circuit formed by
the electromagnetic unit 26, the second clutch member 18 and the
armature 22 then the configuration of the armature 22 would have
relatively less impact on the force exerted between it and the
second clutch member 18, and any dimensional tolerances that exist
at the point where the flux transfer is the most restricted would
play a role in the aforementioned force. This would introduce a
variable into the magnitude of the force that is undesirable.
Another way that the clutch assembly 14 is configured to reduce the
range of magnetic force exerted between the armature 22 and the
second clutch member 18 is to select the material of the armature
22 and to configure the armature 22 to be relatively thin so that
it reaches saturation (or more broadly, so that it reaches at least
a selected level of saturation) of magnetic flux quickly under
conditions which would generally be unconducive to the generation
and transfer of magnetic flux in the aforementioned magnetic
circuit. As a result of this, the magnetic force between the second
clutch member 18 and the armature 22 would vary within an
acceptable range under conditions that would be conducive to
generating a relatively greater magnetic flux in the magnetic
circuit. For example, the range of operating temperatures for the
clutch assembly 14 may be about -40 degrees Celsius to about 120
degrees Celsius. As the temperature increases, the electrical
resistance of the electromagnet 69 and the components that feed
electric current to it increases, and as a result, the current that
reaches electromagnet 69 drops, which in turn reduces the magnetic
flux generated by the electromagnet 69. In addition to a change in
flux that occurs with temperature, the voltage that will be applied
to the electromagnetic unit 26 can vary over some range, such as,
for example, about 9V to about 16V, based on fluctuations that
typically occur in the vehicle's electrical system. In a preferred
embodiment, the armature 22 is configured to be saturated quickly
when operating at a temperature proximate the high end of the
temperature range (i.e. about 120 degrees Celsius in this example)
and when the electromagnetic unit 26 receives a voltage that is
proximate the low end of the voltage range (i.e. about 9V in this
example). As a result, throughout the operating temperature range
and throughout the range of voltages, the magnetic force exerted
between the second clutch member 18 and the armature 22 will vary
within a selected acceptable range.
FIG. 5 show graphs of results from magnetic finite element analyses
showing the force exerted on the armature when there is a 0 gap
(i.e. when the armature 22 is engaged with the second clutch member
18), for configurations of clutch assemblies that are similar to
the clutch assembly 14. The graphs show the relationship between
the force generated (in Newtons) in relation to magnetomotive force
(MMF) as measured in Ampere-Turns. As can be seen in the graph in
FIG. 5, the generated force changes by about 36% (from about 505N
to about 688N) over a range of MMF that varies from 300 AT to 900
AT. This is a much smaller variation than would occur if the
armature 22 were not configured to be substantially saturated under
the worst case scenario for flux generation.
Thus, by selecting suitable materials for the armature and by
configuring the armature in a selected way, (e.g. to be relatively
thin, particularly radially), the force generated on the armature
when the armature is engaged with the second clutch member 18 can
remain within an acceptable range even under relatively wide ranges
of operating conditions. In an exemplary embodiment, the radial
thickness of the armature 22 is about 1.25 mm. In some embodiments,
the armature 22 may be provided with a magnetic flux choke point
that would reduce the magnetic flux through the armature 22 and
would thus promote reaching saturation of the armature 22 under
conditions of poorer magnetic flux generation than would the
armature 22 shown in FIGS. 3a and 3b. The magnetic flux choke point
may be in the form of a reduction in the cross-sectional area of
the armature 22, as can be seen in the sectional views shown in
FIG. 10a-10c. For example, instead of having a rectangular
cross-sectional shape as seen in FIGS. 3a and 3b, the armature 22
may alternatively have a groove 99 in its radially outer surface
shown at 101a (FIG. 10a) that serves to reduce the cross-sectional
area (i.e. the cross-sectional area of the armature 22 in a plane
parallel to the plane P which appears edge-on in the view shown in
FIG. 10a-10c) and to act as a choke point for the magnetic flux.
The groove 99 could alternatively be in the radially inner surface
shown at 101b (FIG. 10b). Alternatively a groove 99a could be
provided in the radially outer surface 101a and another groove 99b
could be provided in the radially inner surface 101b (FIG. 10c). By
providing the reduction in cross-sectional area axially spaced from
the friction engagement surface 82 a magnetic flux choke point can
be provided while still providing a selected surface area to the
friction engagement surface 82. It may be desirable for the
friction engagement surface 82 to have a higher surface area so as
to reduce the wear on that surface (by virtue of spreading the
force of engagement between the armature 22 and the second clutch
member 18 over a selected, large surface area), while providing the
choke point so as to promote saturation under conditions of poor
magnetic flux generation. In the embodiments shown in FIG. 10a-10c,
the reduction in the cross-sectional area of the armature 22 is
achieved by a reduction in the cross-sectional thickness of the
armature 22. Alternatively, the reduction in the cross-sectional
area can be achieved by some other means, such as, for example, by
stamping or otherwise providing a circumferential row of apertures
through the thickness of the armature 22 (i.e. a row of apertures
about the circumference of the armature 22).
As a separate issue from reducing the fluctuation in the magnetic
force exerted on the armature 22 by the second clutch member 18, it
is advantageous to limit the maximum magnetic force that is applied
between the armature 22 and the second clutch member 18, thereby
limiting the frictional force exerted between the armature 22 and
the second clutch member 18. By limiting this frictional force, a
limit is set on the torque that can be transferred through the wrap
spring 20 on the second clutch member 18. More specifically, the
torque that is transmittable through the coils 44 of the wrap
spring 20 to the second clutch member 18 is related to the torque
that is applied between the armature 22 and the second clutch
member 18 (which may be referred to as the energizing torque). This
energizing torque itself depends on the magnetic force between the
armature 22 and the second clutch member 18, the coefficient of
friction therebetween, and the moment arm of the magnetic force
about the axis A. In general the torque that is transmittable at
the coils 44 of the wrap spring 20 can have an exponential
relationship to the energizing torque. In other words, as the
energizing torque increases, the torque transmittable at the coils
44 increases exponentially. Due to the dimensional and material
property tolerances in the components that make up the clutch
assembly 14, the variability of the voltage applied to the
electromagnetic unit 26, the tolerances in the coefficients of
friction between the armature 22 and the second clutch member 18
and between the coils 44 and the second clutch member 18, and other
factors, there is the potential for the energizing torque to vary
dramatically from clutch assembly to clutch assembly and from
situation to situation. If the energizing torque were permitted to
vary unchecked, it could vary by as much as 300% or more depending
on the range of operating conditions the clutch assembly 14 will
have to work in, and depending on the tolerances in the various
components and properties. As a result, if the energizing torque
were simply able to vary unchecked, the wrap spring 20 could be
caused to transmit torques that vary significantly based on the
exponential relationship mentioned above. Thus, in such a case,
either the wrap spring 20 would have to be designed to handle a
very large range of torques, or the torque that could be
transmitted at the coils 44 could become so high that the wrap
spring 20 would be at risk of damage or even failure. However, by
configuring the armature 22 so that it has at least a selected
amount of saturation (e.g. substantially complete saturation) under
the worst case conditions for magnetic flux generation, the
magnetic force that is generated under the best case conditions for
magnetic flux generation will not vary that dramatically from the
flux generated at the worst case conditions. This is a way of
setting a limit on the maximum energizing torque available, which
therefore sets a limit on the maximum torque that will be
transferred at the coils thereby protecting the wrap spring 20 from
failure from transmitting too high a torque, and saving the wrap
spring 20 from having to be overdesigned just to protect it under
scenarios where the tolerances and conditions would have created a
very high energizing torque.
By configuring the armature 22 to have a selected amount of
saturation as noted above under the worst case conditions for flux
generation, when the first clutch member is rotating and the second
clutch member 18 is stationary, and the electromagnetic unit 26 is
energized so as to engage the clutch 14, if the torque required to
drive the second clutch member 18 is too high (i.e. beyond a
selected limit), the armature 22 will slip on the second clutch
member 18. As a result, the angular movement of the second end 42
of the wrap spring 20 will be limited to a selected maximum angle
due to the slippage. The selected maximum angle acts as a limit for
the amount of expansion that is possible for the wrap spring 20 and
therefore acts to limit the force that can be exerted by the wrap
spring 20 on the inner surface 39 of the pulley 18. By limiting
this force, the amount of torque that can be transferred through
the wrap spring 20 to the pulley 18 is limited to a selected
maximum torque.
Another way of reducing the likelihood of an unintentional
expansion of the wrap spring 20 is to control the amount of inertia
that exists in certain components of the clutch assembly 14. One
component in particular whose rotational inertia is selected to be
low is the assembly of the actuator 24 and the armature 22 (which
may be referred to as the actuator/armature assembly. As noted
earlier in this document, the actuator 24 has been described as
being made from a plastic material, such as Nylon 4-6 modified with
Teflon.TM.. Also as noted earlier the armature 22 may be made from
a 1010 steel. Thus, in such an embodiment, a large portion of the
actuator/armature assembly is made from a plastic material (i.e. a
first material having a relatively lower density), and only a
relatively thin band at the radially outer end of the
actuator/armature assembly is made from metallic material (i.e. a
second material having a relatively higher density than the first
material). In at least some embodiments, the aspect ratio of the
armature 22 is such that the radial thickness (shown at T in FIG.
3b) of the armature 22 is smaller than its axial length (shown at L
in FIG. 3b). The mean radius of the armature 22 is selected so that
it provides a selected combination of a selected force between it
and the second clutch member 18 and a relatively low rotational
inertia. The actuator 24 provides the rotational support of the
armature 22 on the first clutch member 16 while having relatively
low weight that is the result of its configuration and of its
material of construction.
By controlling the inertia of this assembly, the actuator/armature
assembly will have a reduced resistance to sudden changes in speed
resulting from accelerations of the engine, for example. By
contrast, if the inertia of the actuator/armature assembly were
relatively high, and the first clutch member 16 underwent a high
acceleration, the inertia of the actuator/armature assembly might
cause such a lag in its rotation, that the wrap spring 20 could
expand radially off the support surface 60 (potentially creating
noise when it returns) and/or generating repetitive stresses in the
wrap spring potentially reducing its life and/or potentially
engaging the second clutch member 18 inadvertently creating other
problems as described earlier.
Because there is so little resistance to movement of the armature
22 and because of the exponential relationship between the
energizing torque and the torque at the wrap spring coils 44, the
energization of the electromagnetic unit 26 may require somewhere
in the range of about 5 W to about 30 W, with a predicted typical
operating range of between about 10 W to about 15 W, of power in
order to generate the magnetic flux needed to drive the armature 22
into the second clutch member 18 with sufficient force to engage
the coils 44 with the second clutch member 18.
Also with respect to inertia, it will be noted that the second
clutch member 18 is made from a relatively thin walled material
(albeit a metallic material at least in some embodiments) so as to
reduce its inertia. Any lightening holes provided in it would have
to be configured to ensure that it can sufficiently transport a
magnetic flux to the armature 22.
With reference to FIG. 3a, the operation of the clutch assembly 14
and the torque flow path are described as follows. With the clutch
14 disengaged, the first clutch member 16 rotates while the second
clutch member 18 (i.e. the pulley 18) remains stationary. One of
the lugs 52 (FIG. 4c) on the first clutch member 16 drive the first
end 40 of the wrap spring 20 and as a result, the wrap spring 20
rotates with the first clutch member 16. The actuator 24, which
sits on the first clutch member 16 is driven to rotate with the
first clutch member 16 by frictional engagement between the
radially inner surface of the actuator 24 and the radially outer
surface of the first clutch member 16.
Energization of the electromagnet 69 draws the armature 22 into
engagement with the pulley 18. Because the pulley 69 is stationary,
the engagement between the armature 22 and the pulley 18 causes the
armature 22, and therefore the actuator 24 to slow down relative to
the first clutch member 16. Because the second end 42 of the wrap
spring 20 is engaged with the drive slot 66 in the actuator 24, the
slowdown of the actuator 24 causes the second end 42 of the wrap
spring 20 to move angularly relative to the first end 40, which in
turn causes the wrap spring 20 to expand radially until the
radially outer surface of the coils 44 engages the radially inner
surface 39 of the pulley 18. Torque is then transferred from the
wrap spring coils 44 to the inner surface 39, thereby driving the
pulley 18.
The embodiment shown in FIGS. 1-4c shows the first clutch member 16
as being directly mounted to the crankshaft 10 of the engine 12. It
will be noted that in some applications, (e.g. where there is no
enough room) the crankshaft 10 could have a first pulley directly
thereon which drives a second pulley on another shaft (i.e. a
jackshaft) via a belt. The clutch assembly 14 could be mounted to
that jackshaft, such that the jackshaft would drive the first
clutch member, and the first clutch member would be selectively
operatively connectable to the second clutch member. Another belt
or the like could run from the second clutch member to a pulley on
an accessory to be driven.
As noted above, an advantage to the embodiment shown in FIGS. 1-4c
is that there are relatively fewer components needed for it to
operate than are used in certain clutches of the prior art. Being
constructed of fewer components reduces the cost of the clutch
assembly 14, reduces tolerance stack-ups, and can increase
reliability since there are fewer components that can fail, as
compared with clutch assemblies of the prior art.
The operation of the clutch assembly 14 may be controlled by a
controller shown at 88 in FIG. 1. Because so little power is needed
to engage the clutch assembly 14, the controller 88 may be directly
connected the electromagnet 69 (FIG. 3a) via electrical conduits
shown at 90 and a MOSFET or the like in the controller 88 may
directly control the current through the conduits 90. This
arrangement is much less expensive than it is for some clutches of
the prior art, such as some friction plate clutches. Those clutches
require a significant amount of power to engage, and less power but
still a significant amount of power to hold the engaged position.
Those clutches would not be controllable directly from a controller
and would thus require the controller to be connected to a relay,
which would be connected to a source of higher electrical current
than can typically be handled by a controller. The relay would then
be controlled by the controller 88 in order to control the current
to the clutch. Conduits would extend from the source of electrical
current (which is ultimately the battery) to the relay and from the
relay to whatever clutch actuation mechanism requires it. Thus,
because of the low power needed to operate the clutch 14, thereby
permitting it to be controlled directly from the controller 88,
there is no need for the aforementioned relay, nor for the conduits
that can carry high current.
The embodiment shown in FIGS. 1-4c selectively drives a pulley
(i.e. second clutch member 18) from a rotating shaft (i.e.
crankshaft 10). Reference is made to FIGS. 6a and 6b, which shows a
clutch assembly 114 that is used to selectively transmit power from
a drive member such as a belt, a timing belt, a chain, a gear or
any other suitable drive member, (not shown) through to a shaft 110
of an accessory 112.
Referring to FIGS. 6a and 6b, the clutch assembly 114 includes a
first clutch member 116 and a second clutch member 118 that are
both rotatable about the axis A, a wrap spring 120, an armature
122, an actuator 124 and an electromagnetic unit 126. The clutch
assembly 114 may be similar to the clutch assembly 14 shown in FIG.
1, and has similar advantages.
The clutch assembly 114 is movable from a disengaged position shown
in FIG. 7a to an engaged position shown in FIG. 7b by transmission
of a magnetic flux from the electromagnetic unit 126 through the
first clutch member 116, the armature 122 and back into the
electromagnetic unit 126. The clutch assembly 114 may be referred
to as being engaged when it is in the engaged position and may be
referred to as being disengaged or unengaged when it is in the
disengaged position.
The first clutch member 116 is driven by a drive member such as a
belt, a timing belt, a chain, a gear or any other suitable drive
member. This drive member may itself be driven by any suitable
means, such as, for example, by a crankshaft of an engine in a
vehicle. In the embodiment shown in FIGS. 6a and 6b, the first
clutch member 116 is a pulley, however it could be some other
member, such as a sprocket, a gear or any other suitable member.
The first clutch member 116 may be made from any suitable material,
such as a suitable steel. The first clutch member 116 may be
rotatably supported on a stationary member 103, which may be
referred to as an electromagnetic unit support member, via one or
more bearing members 128. In this embodiment, there are two bearing
members 128 which are ball bearings however any other suitable type
of bearing member could be used.
Reference is made to FIG. 8, which shows a minor variant of the
embodiment shown in FIGS. 6a and 6b, which uses the same reference
numerals. The electromagnetic unit support member 103 itself mounts
to the accessory housing shown at 105 so as to locate the
electromagnetic unit 126 and the first clutch member 116 about the
axis A. The mounting may be by way of threaded fasteners shown at
107 which pass through the electromagnetic unit support member 103
and thread into accessory housing apertures 109. The
electromagnetic unit support member 103 may also be referred to as
a clutch housing, since it serves to house at least some of the
components of the clutch assembly 114.
The second clutch member 118 is driven by the first clutch member
116 when the clutch 114 is engaged (FIG. 7b), and may be idle when
the clutch 114 is disengaged (FIG. 7a). The second clutch member
118 is connected to the input shaft 110 of the driven accessory
112. In the embodiment shown, the second clutch member 118 is in
the form of a shaft extension.
The second clutch member 118 mounts to the input shaft 110 as
follows: The second clutch member 118 has a shaft mounting portion
180 that has a cross sectional shape that mates with the accessory
input shaft 110 and fixes the second clutch portion 118
rotationally with the accessory input shaft 110. In the embodiment
shown the shaft mounting portion 180 has a splined shape in
cross-section (i.e. it is splined) and it interlocks with
corresponding splines on the input shaft 110. The second clutch
member 118 further includes an input shaft aperture 181, which
receives a threaded fastener 182 that passes through it and threads
into an input shaft aperture 184 in the end of the input shaft 110,
thereby fixing the second clutch member 118 axially on the input
shaft 110.
To install the threaded fastener 182 into the input shaft 110, an
installer passes a fastener tool 190 (e.g. a hex driver) through an
aperture 191 in the first clutch member 116 to engage and drive the
threaded fastener 182 into place in the aperture 184 in the end of
the input shaft 110. After installation of the first and second
clutch portions, a cap 193 can be inserted into the aperture in the
first clutch member 116 into which the position fixing tool 186 and
the fastener tool 190 passed.
In an alternative embodiment shown in FIG. 8, the second clutch
member 118 further includes an anti-rotation element that is
configured to engage a position fixing tool 186 (FIG. 8a) through
the first clutch member 116. When engaged with the position fixing
tool 186, the position fixing tool 186 can be held stationary so as
to fix the second clutch member 118 and therefore the input shaft
110 rotationally. The position fixing tool 186 has a pass-through
aperture 188 that permits an installer to pass a fastener tool 190
(e.g. a hex driver) therethrough to engage and drive the threaded
fastener 182 (FIG. 8) into place in the aperture 184 in the end of
the input shaft 110, while the installer holds the position fixing
tool 186 stationary to prevent rotation of the input shaft 110
while the threaded fastener 182 is being threaded into the aperture
184.
Referring again to FIGS. 7a and 7b, the actuator 124, the armature
122, the wrap spring 120 and the carrier 148 are all engaged with
each other in similar manner to the actuator 24, the armature 22,
the wrap spring 20 and the carrier 48 (FIGS. 3a and 3b), in the
sense that the armature 122 is rotationally operatively connected
with a second end 142 of the wrap spring 120 (i.e. via the actuator
124 in the embodiment shown), the actuator 124 is rotationally
operatively connected with the armature 122, the carrier 148 is
rotationally operatively connected to a first end 140 of the wrap
spring 120 and the carrier 148 is rotationally operatively
connected with the second clutch member 118.
One difference however, is that the actuator 124, the armature 122,
the wrap spring 120 and the carrier 148 are mounted to the second
clutch member 118 and are thus stationary when the clutch 114 is
disengaged, whereas the actuator 24, the armature 22, the wrap
spring 20 and the carrier 48 are mounted to the first clutch member
116 and thus rotate with it even when the clutch 14 is
disengaged.
The actuator 124 is kept in position axially at one end by an
actuator retainer 163 which is fixed to the second clutch member
118 (e.g. by press-fit) and at another end by a carrier retainer
158 which is fixed to the second clutch member 118 (e.g. by
press-fit), and which also engages the carrier 148 to hold it in
place axially.
The carrier 148 may be engaged with the second clutch member 118 in
the same way as the carrier 48 and first clutch member 16 in the
embodiment shown in FIGS. 1-4c, (i.e. by way of lugs and lug
slots). The actuator 124 and armature 122 may engage each other in
the same way as the actuator 24 and the armature 22 in FIGS. 2a and
2b. The first and second ends of the wrap spring 120 may be engaged
by the carrier 148 and the actuator 124 in the same way as the wrap
spring 20 with the carrier 48 and the actuator 24 in the embodiment
shown in FIGS. 1-4c.
The electromagnetic unit 126 is similar to the electromagnetic unit
26 (FIG. 3a) and includes an electromagnet 169 and an
electromagnetic unit housing 170. The electromagnetic unit housing
connects to the electromagnetic unit support member 103 (i.e. the
clutch housing 103) by any suitable means, such as by fasteners,
press-fit, staking or the like.
In this embodiment, when the first clutch member 116 rotates and
the second clutch member 118 is stationary, the magnetic flux in
the first clutch member 116 draws the armature 122 axially into
engagement therewith with sufficient force to drive the armature
122 and the second end 142 of the wrap spring 120 rotationally
about the axis A relative to the first end 140 of the wrap spring
120 so as to radially expand the wrap spring 120 into engagement
with the first clutch member 116 thereby operatively connecting the
first clutch member 116 to the second clutch member 118.
The clutch housing 103, the first clutch member 116 and the bearing
member 128 form at least part of a first clutch portion shown at
192. In this example, the electromagnetic unit 126 also forms part
of the first clutch portion 192. The second clutch member 118, the
armature 122 and the wrap spring 120 form at least part of a second
clutch portion 194. In this example, the actuator 124 and the
carrier 148 also form part of the second clutch portion 194. It
will be noted that there is a radial gap G between the first clutch
portion 116 and the second clutch portion 118. In other words, in
use, when the wrap spring 120 is disengaged with the first clutch
member 116 there is a radial gap between the first clutch portion
192 and the second clutch portion 194.
As a result of this, there is some amount of radial play that is
available between the first and second clutch portions 192 and 194.
This radial play provides the clutch assembly 114 with the
capability to accommodate tolerances in the positions of the
accessory housing apertures 109 in the accessory housing 105
relative to the input shaft aperture 184. In a situation where the
accessory housing apertures 109 and/or the input shaft aperture 181
are out of position relative to each other, when the clutch
assembly 114 is mounted to the accessory 112, the radial gap G
would not have a uniform size throughout its circumference.
However, in such a case, when the electromagnetic unit 126 is
energized, the wrap clutch 120 is flexible and could simply expand
and take on a slight eccentricity relative to the axis A as it
engages the inner surface 139 of the crankshaft adapter 116. As a
result, the clutch assembly 114 can operate with essentially no
change in its performance in situations where there is some lack of
concentricity between the first and second clutch portions 192 and
194. In some accessories, it is expected that there could be a
tolerance in the positions of the housing apertures 105 and the
input shaft aperture 181 that is about 0.25 mm. This tolerance is
easily accommodated by the clutch assembly 114.
By virtue of this capability to accommodate misalignment or lack of
concentricity, the clutch assembly 114 can be mounted as a complete
assembly to the accessory housing 105 and input shaft 110 all at
once. By contrast, some clutch assemblies of the prior art, and in
particular, some clutch assemblies that employ armatures that are
moved across a gap to engage or disengage the clutch assembly are
mounted component by component or subassembly by subassembly and
each component or subassembly is shimmed as necessary to ensure
that strict tolerances in certain gaps are kept. This capability of
being mounted as a complete assembly without the need for shimming
makes the clutch assembly 114 relatively quick and easy to install
as compared to some clutch assemblies of the prior art.
Furthermore, in some embodiments, it may be preferable to grease
certain internal components of the clutch assembly 114 such as the
wrap spring 120. As a result, it is advantageous to be able to ship
the clutch assembly from the clutch assembly manufacturer's
facility with the grease already applied. This is possible in the
embodiment shown in FIGS. 6a and 6b because the clutch assembly 114
can remain in one piece during the installation process. In some
embodiments, the clutch assembly 114 will have no lubrication at
all. In other embodiments the clutch assembly 114 may have a
petroleum-based lubricant or a more-advanced polymer-based,
polymer/ceramic or nanoparticle augmented lubricant instead of a
grease-type lubricant. In yet other embodiments, a lubricious
coating may be applied to the inner surface of the first clutch
member 116 and/or to the wrap spring 120 itself.
The embodiment shown in FIGS. 6a and 6b may be controlled by a
controller similar to the controller 88 to similar advantage.
The armature 122 may have a cross-sectional shape as shown in FIGS.
7a and 7b, or it could have some other cross-sectional shape, such
as any of the shapes shown for the armature 22 in FIG. 10a-10c.
Inclusion of Decoupler
The clutch assemblies 14 and 114 may optionally include an
isolator, an overrunning clutch, or a combination of both, which is
referred to as a decoupler. FIG. 9 shows a decoupler 200 that is
integrated into the clutch assembly 114. The decoupler 200
transfers torque from a pulley 202 to a hub 204. The pulley 202 is
part of the first clutch member 116 and would replace the pulley
197 (FIG. 6a). The pulley 202 could instead be a gear, a sprocket
or any other suitable driven member. The hub 204 in this instance
is also part of the first clutch member 116, and connects to the
hub portion 199 of the first clutch member 116 (FIG. 9 and FIG.
6a). In this embodiment, the hub portion 199 (FIG. 6a) of the first
clutch portion 116 may be modified to have a threaded end 212 (FIG.
9) that is received in a threaded portion 210 of the hub 204
thereby joining them together. The bearing members shown at 214
(which is a ball bearing) and 216 (which is a bushing) may be
provided to center the pulley 202 relative to the hub 204. Thus,
the bearing members 128 shown in FIGS. 7a and 7b may still be
included in this embodiment to center the hub 199 relative to the
member 103. Alternatively some other way of supporting the various
components may be provided so as to reduce the number of bearing
members included.
The decoupler 200 includes a resilient isolation member 206, which
in this exemplary embodiment is a torsion spring, and a one-way
clutch member 208, which in this exemplary embodiment is a wrap
spring. The pulley 202, which is driven by a belt or the like (not
shown), drives the hub 204 through the one-way clutch member 208
and the isolation member 206. The isolation member 206 may be in
the form of a torsion spring that provides some amount of isolation
to the hub 204 from torsional vibrations incurred by the pulley
202. The wrap spring 208 permits the hub 204 to temporarily overrun
the pulley 202 when the pulley 202 is stopped. Instead of providing
the decoupler 200, any other suitable decoupler, isolator, or
one-way clutch member may be used, such as, for example, any of the
structures shown in patent documents U.S. Pat. No. 6,083,130, U.S.
Pat. No. 7,153,227, U.S. Pat. No. 7,618,337, U.S. Pat. No.
7,712,592, U.S. Pat. No. 7,207,910, U.S. Pat. No. 5,722,909 and
WO2011072391A1, all of which are incorporated herein by reference
in their entirety. Alternatively, if it were deemed acceptable for
a particular application, any suitable structures could be used
from the following patents and patent applications: EP01764524A1,
U.S. Pat. No. 7,985,150B2, U.S. Pat. No. 7,708,661B2, U.S. Pat. No.
7,708,661 and US20060240926, all of which are incorporated herein
by reference in their entirety.
In the example shown, the decoupler 200 transfers torque between an
upstream member, namely the pulley 202, and a downstream member,
namely the hub 204. It is alternatively possible to provide an
isolation member 206 only between an upstream member (e.g. the
pulley 202) and a downstream member (e.g. the hub 204) and to omit
the one-way clutch member 208 and associated components. It is
alternatively possible to provide the one-way clutch member 208
only between an upstream member (e.g. the pulley 202) and a
downstream member (e.g. the hub 204) and to omit the isolation
member 206 and associated components.
While the decoupler 200 is shown as being included in the first
clutch member 116, it will be understood that the decoupler 200 (or
alternatively a one-way clutch without an isolator, or
alternatively an isolator without a one-way clutch) could be
included in the second clutch member 118.
While the decoupler 200 is shown as being part of the clutch
assembly 114 it is possible for the decoupler 200 (or an isolation
member only, or a one-way clutch member only) to be incorporated
into the clutch assembly 14.
The isolation member 206 is shown as a torsion spring, however it
will be noted that in some other embodiments the isolation member
could be a resilient polymeric layer (e.g. made of rubber or the
like) that is sandwiched between first and second portions of the
pulley 202.
It will be further noted that the one-way clutch needn't be a wrap
spring clutch, it could alternatively be a sprag clutch a roller
clutch or any other suitable type of one-way clutch.
Reference is made to FIGS. 11a and 11b which are exploded views of
another embodiment of a clutch assembly shown at 314. The clutch
assembly 314 may be similar to the clutch assembly 14 (FIG. 1) but
with several differences which are described hereinbelow. In
general, the clutch assembly 314 is, in at least some embodiments,
configured to permit the transfer of a high amount of torque
between a first clutch member 316 (which may be a crankshaft
adapter that mounts to a crankshaft 10 shown in FIG. 1) and a
second clutch member 318 (which may be a pulley), while also
including features to ensure that the engagement of the clutch
assembly 314 is controlled (i.e. is not very abrupt unless desired)
and that disengagement occurs reliably. It will be noted that, for
readability, the first clutch member 316 may be referred to as the
crankshaft adapter 316, and the second clutch member 318 may be
referred to as the pulley 318. It will be understood, however, that
in many instances it is intended solely for readability and that
any suitable first clutch member and any suitable second clutch
member could be used.
Parts of the clutch assembly 314 that are similar in function to
parts of the clutch assembly 14 will have similar reference
numerals (amended to include a leading `3`). Accordingly, the
clutch assembly 314 includes the crankshaft adapter 316, the pulley
318 which is supported on the first clutch member by a bearing 328,
a retainer 358 that holds a carrier 348 that holds a first end 340
of a wrap spring 320, an armature 322 that holds a second end 342
of the wrap spring 320, an actuator 324, an electromagnetic unit
326 that includes an electromagnet 369 and an electromagnetic unit
housing 370 that holds the electromagnet 369, and a control system
388. In the embodiment shown, the first clutch member 316 is a
crankshaft adapter that mounts to and is driven by the crankshaft
10 (FIG. 2a). A second pulley (analogous to pulley 34 in the
embodiment shown in FIG. 1 is not shown in FIGS. 11a and 11b)
however such a pulley could optionally be provided.
As with the clutch assembly 14 shown in FIG. 1, energization of the
electromagnet 369 causes the armature 322 to engage the pulley 318.
Because the armature 322 holds the second end 342 of the wrap
spring 320, such engagement between the armature 322 and the pulley
318 causes a shift in the phase angle between the armature 322 and
the crankshaft adapter 316, which causes the wrap spring 320 to
expand radially and engage the second clutch member 318 thereby
engaging the crankshaft adapter 316 with the second clutch member
318. Deenergization of the electromagnet 369 causes the wrap spring
320 to contract radially away from the pulley 318 so as to
disengage the crankshaft adapter 316 from the second clutch member
318.
Use of Return Spring on Armature
The armature 322 is provided with a return spring to bring the
armature 322 away from the pulley 318 when the electromagnet 369 is
deenergized.
As with the armature 22 shown in FIGS. 3a and 3b, the armature 322
(FIG. 11c) is movable between a first position in which the
armature 322 is disengaged from the pulley 318 and a second
position in which the armature 322 is drawn into engagement with
the pulley 318 by a magnetic circuit that passes through the pulley
318 itself. A feature of the armature 322, however, is that it is
biased towards the first position by an armature biasing member
365a, which may be, for example, a leaf spring. The armature
biasing member 365a is preferably configured to apply a relatively
small (e.g. 10N) force on the armature 322 to draw the armature 322
away from the pulley 322 after deenergization of the electromagnet
369. As a result, the armature 322 is inhibited from remaining in
contact with the pulley 318 after deenergization of the
electromagnet 369 due to residual magnetism in the pulley 318 and
armature 322 and/or due to sticking as a result of grease that
could be present between them. By eliminating such contact,
premature wear on the armature 322 that could result from such
contact is avoided.
During use of the clutch assembly 314 the electromagnet 369 may be
energized cyclically to control the armature 322 using pulse width
modulation as will be described further below. Depending on the
frequency of energization, a harmonic may be imparted to the leaf
spring 365a such that it is urged to vibrate. To inhibit this
vibration, behind the leaf spring 365a (i.e. to the left of the
leaf spring 365a in the view shown in FIG. 11c) is a rigid backing
plate 365b that abuts the leaf spring 320 to limit movement of the
leaf spring 320, thereby helping to stabilize the leaf spring 365a
in the event it vibrates.
The armature 322 is connected to the leaf spring 389 via a
plurality of rivets 323 that pass through tabs 325 (which are more
clearly shown in FIGS. 11a and 11b) on the armature 322. The rivet
323 shown in FIG. 11c is shown as extending through the tabs 325
and the leaf spring 365a and backing plate 365b but is shown prior
to spreading of its open end to form a mushroom head.
With continued reference to FIG. 11c, the leaf spring 365a and
backing plate 365b are mounted to the actuator 324. Such mounting
may be by mechanical fasteners 359 (e.g. rivets) (FIG. 11a) which
pass through the actuator 324 and further pass through apertures
367a on the leaf spring 365a and apertures 367b on the backing
plate 365b.
Referring again to FIG. 11c, the actuator 324 may be made from a
(preferably non-magnetic) metal such as aluminum. A bushing 371,
which may be made from any suitable material such as DU.RTM.
provided by GGB North America LLC of New Jersey, USA, is mounted in
the actuator 324 and permits the actuator 324 to be rotatably
mounted to the crankshaft adapter 316. The actuator 324 has a slot
366 therein that holds the second end 342 of the wrap spring 320.
As in the embodiment shown in FIGS. 1-4c, when the armature 322 is
drawn into engagement with the pulley 318 friction from the
engagement slows the armature 322, and therefore the actuator 324,
relative to the crankshaft adapter 316. As a result, the second end
342 of the wrap spring 320 rotates by some amount relative to the
first end 340 in a direction that causes the wrap spring 320 to
expand radially into engagement with the inner surface shown at 339
of the pulley 318. As can be seen, the inner surface of the pulley
318 is the inner surface of the sleeve 412.
As shown in FIG. 11c, the actuator 324 may include a `flinger` 329,
which is a lip that extends radially outwards towards the wall of
the pulley 318. The flinger 329 acts to throw debris radially
outwards centrifugally by virtue of the rotation of the actuator
324 if such debris should collect in the clutch assembly 314 during
use. The flinger acts as a rough seal by having its distal edge
shown at 329a be positioned proximate the wall of the pulley 318 so
as to inhibit the entry of debris into the region of the clutch
assembly 314 where the wrap spring 320 resides.
An actuator retainer 363 is provided on the crankshaft adapter 316
to prevent withdrawal of the actuator 324 away from the pulley 318.
The actuator retainer 363 may be a separate ring that is press-fit
or welded or otherwise joined to the crankshaft adapter 316. The
actuator retainer 363 acts as a thrust bearing to support the
actuator 324 during use of the clutch assembly 314. The bushing 371
may be L-shaped so as to have a portion that acts between the
actuator 324 and the actuator retainer 363 to reduce friction
therebetween as the actuator 324 rotates relative to the actuator
retainer 363 and crankshaft adapter 316. Aside from the provision
of the actuator retainer 363 to limit the maximum distance of the
actuator 324 from the pulley 318, the actuator 324 may otherwise be
permitted to float axially to some extent over a small distance
along the crankshaft adapter 316.
The wrap spring 320 may be provided with a relatively large number
of coils 344, also referred to as turns, so as to permit a
relatively large amount of torque to be transferred through the
wrap spring 320 to the pulley 322. However, in general, the
abruptness of the engagement between a wrap spring and the driven
component (in this case the pulley 318) increases as the number of
coils on the wrap spring increases. Accordingly, it is typical to
limit to number of coils on a wrap spring in order to control the
dynamic torque that will be transferred to the pulley 318 and thus
control the stresses incurred by the various components. However it
is desirable to provide a large number of coils in applications
where the clutch assembly is needed to handle high torque.
Alternatively, however, a wrap spring 320 with a relatively small
number of coils may also be used in the clutch assembly 314. For
example, the wrap spring 320 could have three coils, or even fewer
in some embodiments.
The clutch assembly 314 provides a wrap spring 320 with a large
number of coils 344 so as to permit high torque transfer, but also
includes structure to permit control over the abruptness of the
engagement of the wrap spring 320 with the pulley 318.
Use of Sensors and PCB in Electromagnet Housing
Referring to FIGS. 19a and 19b, the low power requirements of the
clutch assembly 314 that result from providing the magnetic circuit
through the pulley 318 itself permit a control system 388 to be
provided with it. The control system 388 is used for controlling
the rate of engagement of the wrap spring 320 with the pulley 318,
and also to control the amount of torque that can be transferred
through the wrap spring 320 to the pulley 318 during operation. The
control system 388 includes a PCB with processor and memory,
sensors and a driver, so as only to require a connection to the
vehicle's data bus (in embodiments wherein data is sent to and from
the control system 388 via a wired connection), and a power
connection to the vehicle's battery (not shown). In some
embodiments however, data may be sent to and from the control
system 388 wirelessly via any suitable wireless protocol such as
Zigbee. Also, in some embodiments, the control system may be housed
in the housing shown at 370 for the electromagnet 369. Room for the
control system exists in part because the size of the electromagnet
369 can be small compared to the electromagnet used on some typical
clutches which require more power to operate, and in part because
the size (the axial length specifically) of the electromagnet
housing 370 may be dictated to some extent so as to provide a
certain amount of axial overlap with the pulley 320. The smaller
size of the electromagnet 369 is enabled because of the greater
efficiency of the magnetic circuit in the clutch assembly 314 than
in some clutch assemblies of the prior art. For example, in some
clutch assemblies the magnetic flux travels from the electromagnet,
across a gap to a pulley, across the pulley (axially), across
another gap to a friction plate, radially across the friction
plate, back across the second gap to the pulley again, and then
back across the first gap to the electromagnet. The many air gaps
necessitate the use of a relatively large electromagnet.
Additionally, some of the magnetic flux short circuits through the
pulley radially and back into the electromagnet and does not reach
the friction plate again necessitating a large electromagnet to
account for the lost power. By comparison, the magnetic circuit of
the clutch assembly 314 (and the other clutch assemblies described
herein) has an air gap between the electromagnet housing 370 and
the pulley from the pulley 320 to the armature 322 and from the
armature 322 back into the housing 370, thereby reducing the number
of air gaps that must be crossed. As a result a smaller
electromagnet can be used to drive the magnetic circuit, and
provides a space inside the housing 370 for the PCB 391 and other
components of the control system 388.
Providing the control system 388 in this way permits the clutch
assembly 314 to be more likely to be capable of incorporation into
a vehicle using the existing ECU for the vehicle. Also, this
arrangement keeps the clutch assembly 314 relatively compact.
The armature 322 and the pulley 318 of the clutch assembly 314 each
have a plurality of teeth 322a and 318a respectively thereon. Two
Hall effect sensors 389 (one sensor 389a and one sensor 389b)
detect the passage of the teeth 322a and 318a respectively. The
control system 388 includes a PCB (printed circuit board) 391 that
contains a processor 393, a memory 395 and a driver 397 for
providing current to the electromagnet 369. The processor 393
receives signals from the sensors 389a and 389b and can determine,
among other things, the speeds and accelerations of the pulley 318
and armature 322 and the phase angle between the crankshaft adapter
316 and the armature 322. Using this data, the control system 388
can control the expansion of the wrap spring 320 and therefore the
engagement force between the wrap spring 320 and the inner surface
339 of the pulley 318. It will be understood that the armature 322
is connected rotationally with the actuator, and so the actuator
324 and the armature 322 have the same rotational speed,
acceleration and phase angle.
As with the other embodiments shown and described, by generating a
magnetic loop through the armature 322, the electromagnet housing
370 and the pulley 318, the amount of power required is relatively
small. This permits the use of a relatively low power driver (i.e.
driver 397), which, in turn, facilitates mounting the driver 397
directly on the PCB 391 and eliminates the need for additional
relays as described above in relation to the embodiment shown in
FIGS. 1-4c. Additionally, the sensors 389a and 389b may both be
mounted directly on the PCB 391. Providing the PCB 391, processor
393 and memory 395 as part of the clutch assembly 314, with the
driver 397 and sensors 389a and 389b mounted directly on the PCB
391 eliminates all of the wiring (and associated electrical
connections) that would otherwise be provided between these
components if they were remote from each other. By eliminating
wiring between the sensors 389a and 389b and the PCB 391, the
potential for external noise to interfere with the sensor signal to
the PCB 391 is eliminated. Additionally, the potential for poor
signal transmission resulting from bad electrical connections is
greatly reduced since the number of individual electrical
connections between the PCB 391 is lower than it would be for a
remotely located control unit. Also, by providing the driver 397
and the rest of the control system 388 with the clutch assembly
314, each control system 388 can be calibrated specifically for use
with the components of the specific clutch assembly 314 that it
forms part of. By contrast, with some other clutch assemblies, the
driver and the control system are part of the ECU for the engine
and are thus separated from the clutch assembly itself. As a
result, during vehicle assembly, it is entirely possible for the
engine ECU to require replacement if it is found to be defective,
in which case, the calibration may have to be redone anew upon
installation of the new ECU. By mounting the control system 388
directly with the clutch assembly 314, the calibration can be done
and if the engine ECU requires swapping out, it will not affect the
calibration of the control system 388. In addition to the above,
eliminating the electrical connections that are necessary when
connecting two remote components with wires eliminates many
potential sources of failure, since each connection represents a
potential weak point that can be severed during use (or poorly
manufactured). As yet another advantage, some manufacturers
manufacture families of engines with different units having
different levels of performance. For example, an engine family may
include models that lack a supercharger and models that include a
supercharger. The engine ECU is typically unchanged across the
engine family. If the engine ECU also housed the driver and other
components for the clutch assembly, then it would typically be
provided by the manufacturer for all models of the engine,
including those that lack a supercharger, resulting in additional
cost with no benefit. By contrast, by providing the control system
388 with the clutch assembly, the clutch assembly 314 is
self-contained, and the driver and control system components need
only be provided on engines that include the supercharger.
Because of the low power requirement of the electromagnet 369, the
driver 397 may draw relatively little power from the vehicle's
battery (not shown). The relatively low power draw results in
relatively low heat generation by the driver 397. Additionally, the
electromagnet 369 itself also generates relatively little heat as
compared to a larger electromagnet that is used on some clutch
assemblies such as some friction plate clutch assemblies. For
example, the current draw for some clutch assemblies is between 4
and 5 Amps. By contrast, the current draw on some embodiments of
the clutch assembly 314 may be about 1 Amp. Assuming the same
voltage source is used (the vehicle battery), the power consumed by
the clutch assembly 314 is, in some embodiments, significantly
lower than some other clutch assemblies. Because of the reduced
size of the electromagnet 369 compared to higher power
electromagnets needed for other clutch assemblies, and because of
the reduced heat generation by both the electromagnet 369 and the
driver 397, it is possible to locate the control system 388 within
the electromagnet housing 370 without generating an unduly large
amount of heat in the housing 370. By contrast, in some other
clutch assemblies, even if there were room to store the control
system within the electromagnet housing, so much heat would be
generated that it could result in a reduced operating life for some
of the control system components. As shown in FIGS. 19a and 19b,
the PCB 391 is positioned in the electromagnet housing 370. By
positioning the control system 388 (and the PCB 391 in particular)
in the housing 370, it is protected from mechanical damage and from
the elements to some extent without the need to provide a separate
dedicated enclosure for it. This reduces the cost associated with
it and reduces the amount of room occupied by the clutch assembly
14.
While it is shown for the control system 388 to be housed in the
electromagnet housing 370, it will be understood that the control
system 388 could, in some embodiments, be housed in a separate,
dedicated housing.
Aside from the above noted components, the PCB 391 may have
thereon, a voltage monitoring unit 381, a CAN message translator
383, a PWM module 385, and a current monitoring unit 387. The CAN
message translator 383 is used to receive and send messages from
and to the vehicle's CAN bus 399. The PWM module 385 is used to
generate the PWM signal that is sent to the driver 397 to control
the current sent to the electromagnet 369.
While the CAN message translator 383 is shown as having a wired
connection to the vehicle's CAN bus in FIG. 13, it will be noted
that it is possible to provide a wireless communications device
instead of the wired CAN message translator 383, as mentioned
above.
Instead of a CAN message translator, an analogous device configured
to communicate over a different type of network (e.g. LIN) may be
used.
The control system 388 may include any suitable additional
components to assist in the deenergization of the electromagnet 369
in a controlled way, such as, for example, a Zener diode and/or
other diodes as shown in FIG. 13.
Control Algorithm
The control algorithm used by the control system 388 is illustrated
as a block diagram in FIG. 14a which illustrates the inputs and
outputs to different portions of the algorithm, and as a flow
diagram in FIG. 14b to show the method steps taken when carrying
out the algorithm. The algorithm may also be referred to as a
method. Referring to FIG. 14b, the method is shown at 600 and
begins at 602. At step 604, the control system 388 receives
commands from the engine ECU (not shown) as to whether to engage
the clutch 314 or not, via the vehicle's data bus (e.g. a CAN bus)
shown at 399. At step 606 selected parameters are checked, relating
to the state of the engine, the speed of the pulley 318, the
voltage at the vehicle's battery (not shown) and any other suitable
parameters. At step 608, the control system 388 determines whether
it is okay to engage the clutch 314, based on the parameters that
were checked. For example, if the battery did not have sufficient
charge to maintain the necessary current to the electromagnet 369
for a sufficient amount of time, then the processor 383 may
determine that it is not okay to engage the clutch. If the
determination is `no`, then at step 610, the control system 388
reports to the engine ECU (not shown) that the clutch 314 will not
be engaged.
The control algorithm used by the control system 388 permits fast
or slow engagement of the wrap spring 320 with the pulley 318 when
desired based on a set of selected parameters. The command from the
engine ECU may also specify whether a fast or slow engagement is
needed. The control algorithm incorporates closed loop feedback so
as to permit control of the engagement of the wrap spring 320. Such
feedback may be provided from dedicated sensors, or from data from
preexisting sensors in the vehicle, received via the vehicle's data
bus.
The control system 388 can provide a relatively fast ramp up time
for the engagement between the wrap spring 320 and the pulley 318
in some situations (which is illustrated in FIG. 12a), and can
provide a relatively long ramp up time for the engagement in some
situations (illustrated in FIG. 12b).
In general, the current supplied to the electromagnet 369 results
in an electromagnetic force between the armature 322 and pulley 318
and thereby controls the force of engagement between the armature
322 and the pulley 318 which is proportional to the torque exerted
between the armature 322 and the pulley 318 (via frictional
engagement between their mutually facing surfaces shown at 382 and
380 respectively in FIG. 11c). The torque applied retards the
armature 322 relative to the crankshaft adapter 316. Because the
second end 342 of the wrap spring 320 is captured by the actuator
324, the retardation of the armature 322 results in a radial
expansion of the wrap spring 320. This is resisted by the restoring
force of the wrap spring 320 which biases the wrap spring 320 back
towards an unexpanded state. As the wrap spring 320 expands, the
restoring force increases. Thus from any given position, an
increase in the torque applied to the wrap spring 320 results in an
increase in the amount of expansion until the restoring force
results in a resistive torque that matches the torque applied it.
Based on the above, it can be seen that the force of engagement
between the armature 322 and the pulley 318 is proportional to the
amount of expansion of the wrap spring 320. Furthermore, because
the current applied to the electromagnet 369 is proportional to the
force of engagement, then it will be understood that the current is
proportional to the amount of expansion of the wrap spring 320. It
will also be noted that the amount of expansion of the wrap spring
320 determines the angular displacement (also referred to as the
phase angle) between the first end 340 and the second end 342.
Thus, by controlling the current to the wrap spring 320, the phase
angle between the first and second ends 342 of the wrap spring 320
can be controlled and the amount of expansion of the wrap spring
320 can be controlled.
The control algorithm 600 shown in FIGS. 14a and 14b may have three
stages of operation. In Stage 1, the current to the electromagnet
369 is controlled so as to bring the actuator 324 and armature 322
to a selected relative angular displacement or phase angle relative
to the crankshaft adapter 316. This selected phase angle may be
represented by set point SP1, which may be a value that is stored
in memory 395. SP1 may be selected to be the phase angle at which
the wrap spring 320 has expanded just enough to engage the inner
surface of the pulley 318. In some embodiments, SP1 may be a fixed
value that is stored in memory 395. At step 612 (FIG. 14b), the
values for the actuator speed and the crankshaft speed are
obtained, and the actual phase angle is determined using these
values. When the wrap spring 320 reaches the selected phase angle
SP1, friction and thus torque transfer between the coils of the
wrap spring 320 and the pulley 318 are low. In some instances, SP1
may be somewhere in the range of about 30 to about 50 degrees. In
general, for a wrap spring that is longer (i.e. that has more
coils), the greater the phase angle that will be needed to cause it
expand by a selected amount. As shown in FIG. 14b, the process of
reaching set point 1 is achieved by a loop that includes steps 614,
616 and 618. At step 614 the control system 388 (FIG. 11c) checks
whether the set point SP1 has been reached. If the set point SP1
has not been reached then step 616 (FIG. 14b) is carried out in
which the control system 388 (FIG. 11c) generates a new value for
the current to send to the electromagnet 369. At step 618 (FIG.
14b) the current sent to the electromagnet 369 (FIG. 11c) is
adjusted towards the new value. Control is then sent back to step
614 (FIG. 14b) where the control system 388 checks if the actual
phase angle of the actuator 324 (FIG. 11c) matches the set point
value SP1. If it is determined that the phase angle does equal the
value for SP1, then the control system 388 sends control to step
620 (FIG. 14b), at which point stage 2 of the algorithm 600
begins.
At step 616 a PID control formula may be used. Feedback for the PID
control formula may be provided by the integral of the speed
difference between the actuator 324 (FIG. 11c) and the crankshaft
adapter 316, which will be understood to be indicative of the phase
angle between the armature 322 and the crankshaft adapter 316.
Because the crankshaft adapter 316 is mounted on the crankshaft 10
(FIG. 2a), the crankshaft speed is the same as the crankshaft
adapter speed. The crankshaft speed may be obtained by any suitable
means, such as by communication with existing controllers on the
vehicle via the vehicle's CAN bus 399 (FIG. 13). The value for the
current generated from step 616 (FIG. 14b) is transmitted to the
PWM module 385 (FIG. 13), which outputs a pulse width modulated
signal to the driver 397, which in turn controls the current to the
electromagnet 369 based on the signal from the PWM module 385. Once
the selected phase angle SP1 is achieved, stage 2 of the algorithm
is carried out.
In stage 2 of the algorithm the expansion of the wrap spring 320
(FIG. 11c) is increased in a controlled way to control the
acceleration of the pulley 318. In this stage another PID formula
may be employed (or alternatively any other control formula). The
set point for this control routine is represented by SP2 and is an
acceleration value. Feedback is provided for this control routine
by determining the actual acceleration of the pulley 318 and
comparing it to the set point SP2. The actual acceleration may be
determined by measuring the speed of the pulley 318 using the Hall
effect sensor 389b (FIG. 19b), and by taking the derivative of the
measured speed. Once a selected amount of torque is being
transferred from the wrap spring 320 (FIG. 11c) to the pulley 318,
the algorithm may proceed to Stage 3. The expansion of the wrap
spring 320 is still controlled by controlling the phase angle
between the armature 322 and the crankshaft adapter 316 and so the
phase angle is also an input to the algorithm during stage 2. In
other words, in stage 2, the control system 388 controls the phase
angle between the armature 322 and the crankshaft adapter 316 to
achieve a selected acceleration for the pulley 318. The set point
SP2 may ultimately be derived by receiving a torque value from the
engine ECU (not shown) and then using that value to find the set
point SP2 from a look up table stored in memory 395. Stage 2 of the
algorithm is carried out until the pulley 318 has reached the
engine speed, (i.e. until the crankshaft adapter 316 and the pulley
318 have the same speed). Once the control system determines that
pulley speed matches the engine speed, stage 3 of the algorithm is
carried out.
With reference to FIG. 14b, stage 2 includes a loop that includes
steps 620, 622 and 624. At step 620, it is determined whether the
pulley speed matches the crankshaft speed. If it does not, step 622
is carried out in which a control formula is applied (e.g. a PID
control formula) to provide a new value for the current to the
electromagnet 369 (FIG. 11c) in order to adjust the acceleration of
the pulley 318 towards the set point SP2. At step 624 (FIG. 14b),
the current sent to the electromagnet 369 (FIG. 11c) is adjusted
towards the new value. Control is then sent back to step 620 (FIG.
14b) where the control system 388 (FIG. 11c) checks whether the
pulley speed matches the crankshaft speed. If it is determined that
the pulley speed matches the crankshaft speed, then the control
system 388 sends control to step 626 (FIG. 14b), at which point
stage 3 of the algorithm 600 begins.
In stage 3 of the algorithm, the control system 388 maintains the
holding torque achieved at the end of stage 2 so as to prevent
slippage between the wrap spring 320 and the pulley 318. In this
stage, the control system 388 obtains the speeds of the pulley 318
and the crankshaft 10 (FIG. 2a) and compares them to determine if
any slippage is occurring between them, which would be indicative
of slippage between the wrap spring 320 and the pulley 318. If
slippage is detected, the control system 388 increases the current
to the electromagnet 369 to increase the expansion of the wrap
spring 320, thereby increasing the force of engagement between the
wrap spring 320 on the pulley 318 so as to increase the amount of
torque that can be transferred to the pulley 318 without slippage.
Alternatively, if no slippage is detected, the control system 388
may reduce the current by some amount while continuing to determine
whether there is slippage. In this way, the control system 388
dynamically adjusts the current sent to the electromagnet 369 to
keep the current to a relatively low level while ensuring that the
torque requirements for the clutch assembly 314 are being met. The
three stages of operation of the algorithm are identified in FIGS.
12a and 12b (shown at Stage 1, Stage 2 and Stage 3). References to
input speed and output speed in these two figures are intended to
refer to the speed of the crankshaft and of the pulley 318
respectively.
With reference to FIG. 14b, stage 3 includes a loop that includes
steps 626, 628, 630 and 632. At step 626, the amount of slippage is
determined between the pulley (FIG. 11c) and the crankshaft 10
(FIG. 1). At step 628 it is determined whether the determined
slippage value constitutes slippage (i.e. it is determined whether
it is non-zero). If it is non-zero, step 630 is carried out in
which a control formula is applied (e.g. a PID control formula) to
provide a new value for the current to the electromagnet 369 (FIG.
11c) in order to adjust the engagement force between the wrap
spring 320 and the pulley 318 so as to reduce the amount of
slippage towards zero. At step 632 (FIG. 14b), the current sent to
the electromagnet 369 (FIG. 11c) is adjusted towards the new value.
Control is then sent back to step 626 (FIG. 14b) where the control
system 388 (FIG. 11c) checks whether any slippage is occurring. If
it is determined that there is no slippage (i.e. the pulley speed
matches the crankshaft speed), then at step 628 (FIG. 14b) control
is sent back to step 626 (FIG. 14b). Operation in stage 3 continues
until the electromagnet 369 (FIG. 11c) is deenergized.
Thus by providing the algorithm 600 described above, the clutch
assembly 314 can be configured to permit high torque transfer to
the pulley while still permitting the stresses and dynamic torques
incurred by the components of the assembly 314 to be limited in
situations where such limits are beneficial and to also permit very
short ramp up times for the pulley 318 in situations where short
ramp up times are desired. Short ramp up times, for example, may be
desired during passing maneuvers where the pulley 318 is being used
to drive a belt that ultimately drives a supercharger, thereby
providing power quickly to the vehicle. Longer ramp up times may be
desired where a short ramp up time is not needed and would produce
unacceptably high levels of noise or other problems such as high
stresses.
It will be noted that the use of the algorithm described above is
not limited to a clutch assembly with a wrap spring with many
coils, nor to a clutch that drives a magnetic loop through the body
of the pulley. It is applicable to other types of clutch that
incorporate a wrap spring.
It will be further noted that the algorithm need not include all
three stages. For example, the algorithm could start with stage 2,
whereby the control system 388 continues to control the current to
the electromagnet 369 until a selected acceleration is achieved
(i.e. until the acceleration reaches SP2). In such a scenario there
would simply be an initial period during which the control system
388 sends current to the electromagnet but with no resulting
acceleration in the pulley 318. The control system 388 could
compensate for this in some suitable way such as by delaying the
application of Stage 2 by a certain period of time to give the
armature 322 time to approach the pulley 318. While it is described
that stage 1 may be effectively omitted, it will be noted that the
algorithm could contain any one or any two of the three stages
independent of whether the other stages are included. Preferably
all three stages are included, however.
While two speed sensors 389a and 389b are described it will be
noted that at least one of the stages, and in some instances all of
the stages of the above described algorithm may be carried out even
if one or both of the sensors 389a and 389b are omitted. For
example, stage 2 could be carried out solely using the pulley speed
sensor 389b (i.e. without an armature speed sensor). For example,
the control system 388 could use the pulley speed sensor 389b to
determine the pulley acceleration and could control the current to
the electromagnet 369 to achieve the selected acceleration. The
control system 388 can then compare the pulley speed to the
crankshaft speed and can stop stage 2 when the two speeds match.
Alternatively, if a speed sensor is provided on the driven
accessory (e.g. the supercharger) then that speed sensor can be
used to determine the speed of pulley 318 based on a ratio of the
sizes of pulley 318 and the accessory drive pulley. Thus, stage 2
could be carried out without either of the sensors 389a and
389b.
High Torque Capacity Pivoting Carrier
Another feature of the clutch assembly 300 is that the carrier 348
and its engagement with the retainer 358 permit the transfer of
high torque without deformation of these two components and reduce
the stresses in these components relative to a typical carrier. To
achieve these advantages, the carrier 348 is, in at least some
embodiments, pivotable relative to the retainer 358.
The carrier 348 is shown more clearly in FIGS. 15a and 15b. The
carrier 348 may be made from a metallic material such as a suitable
steel, such as a 1045 carbon steel or a 4340 alloy steel, although
other materials are contemplated.
The carrier 348 includes a spring receiving slot 402 for receiving
the first end 340 of the wrap spring 320 such that a helical end
face 405 of the wrap spring 320 engages a drive wall 406 at the end
of the slot 402. The first end 340 of the wrap spring 320 (and the
slot 402) may have any suitable shape such as an arcuate shape. The
wrap spring first end 340 may be press-fit into the slot 402 so
that there is sufficient friction between the first end 340 and the
slot 402 to prevent withdrawal of the first end 340 from engagement
with the drive wall 406 during moments when the pulley 318 overruns
the crankshaft 10. The wrap spring first end 340 may alternatively
be welded into the slot 402.
The carrier 348 is positioned in an opening 409 in the retainer
358. The carrier 348 has a torque transfer surface 407 at a first
end 411, which engages a torque transfer surface 408 on the
retainer 358 (which is at a first end of the opening 409). As in
other embodiments, the retainer 358 may be press-fit onto the
crankshaft adapter 316 so as to co-rotate with the crankshaft
adapter 316. As a result of this arrangement, torque from the
crankshaft adapter 316 is transferred to the carrier 348 via the
torque transfer surfaces 407 and 408, and from the carrier 348 to
the wrap spring 320 via engagement of the drive wall 406 with the
helical end face 405 of the wrap spring 320. This is a different
arrangement from that which is shown in FIGS. 4b and 4c in which
the lug 52 on the first clutch member 16 directly engages the
helical end face 20a of the wrap spring 20. The torque transfer
surfaces 407 and 408 (FIG. 16) have larger surface areas of contact
with one another compared to the surface area of contact between
the lug 52 (FIG. 4c) and the helical end face shown at 20a (FIG.
4a) of the wrap spring 20 that the lug 52 engages. The larger
surfaces areas permit the retainer 358 to transfer a large amount
of torque into the wrap spring 320 without a risk of damage to the
retainer 358 from the helical end face 405 of the wrap spring 320.
While the surface area of contact between the carrier 348 and the
helical end face 405 of the wrap spring 320 may be similar to that
between the lug 52 (FIG. 4c) and the end face of the wrap spring 20
(and smaller than that between surfaces 407 and 408), the carrier
348 may be made from a suitably strong material to avoid
deformation during torque transfer relatively inexpensively as
compared to forming the entire retainer 358 out of a similar
material.
As can be seen in FIGS. 16, 17, 18a and 18b, the surfaces 407 and
408 may be arcuate so as to permit pivoting of the carrier 348
relative to the retainer 358. When the wrap spring 320 is radially
contracted the carrier 348 may have a first orientation relative to
the retainer 358 as shown in FIG. 18a. When the wrap spring 320 is
radially expanded so as to engage the radially inner surface 339 of
the pulley 318 the carrier 348 can pivot radially outwardly to a
second orientation relative to the retainer 358 as shown in FIG.
18b. As can be seen, by permitting the pivoting of the carrier 348,
only a relatively small length of the wrap spring 320 is
unsupported between the carrier 348 and the surface of the pulley
318. This length is shown at LU. This unsupported length LU is
smaller than it would be if the carrier 348 were fixed in position
and unable to pivot. By providing a relatively small unsupported
length of wrap spring, the wrap spring 320 is able to transmit
relatively large torques without buckling.
Additionally, it will be noted that, during use, the torque exerted
by the retainer 358 on the end face 405 of the wrap spring 320 does
not act at exactly the same radius as the torque acting between the
wrap spring 320 and the pulley 318, which acts on the radially
outer surface of the wrap spring coils. If the carrier 348 was a
fixed (i.e. non-pivoting) carrier and remained in the position
shown in FIG. 17a, the outer corner (shown at 418) of the slot 402
that holds the wrap spring 320 could impinge upon (and therefore
stress) the wrap spring 320 where the wrap spring 320 leaves the
slot 402 and extends outwardly towards the inner surface 339 of the
pulley 318. As a result a stress could be incurred on the first end
340 of the wrap spring 320 where it bends around the outer corner
418. Such a stress can be cyclical as the wrap spring repeatedly
stops and starts transferring torque from the crankshaft 10 to the
pulley 318, raising the risk of fatiguing the first end 340 of the
wrap spring 320. By permitting the carrier 348 to pivot, the
carrier 348 can pivot so that the corner 418 of the slot 402 is
moved out of the way of the first end 340 to permit the first end
340 to extend radially outwards towards the enlarged diameter
without impinging on the carrier 348. While it may be preferred in
some embodiments to provide a pivoting carrier 348, it will be
noted that in some embodiments a fixed carrier may nonetheless be
used where the stresses incurred by the wrap spring 320 do not lead
to an unacceptably short service life.
Thus, the pivoting capability of the carrier 348 permits larger
torques to be transferred by the clutch assembly 314. The torque
transfer surface 407 may, for example, be a convex, generally
part-cylindrical surface and the complementary torque transfer
surface 408 on the retainer 358 may, for example, be a concave,
generally part-cylindrical surface, although other shapes for the
surfaces 407 and 408 may be used. Providing arcuate and preferably
part-cylindrical torque transfer surfaces ensures that the forces
transmitted between the retainer 358 and the carrier 348 are
transmitted across a relatively large surface area even when the
carrier 348 is pivoted in different orientations.
When the electromagnet 369 is deenergized so that the wrap spring
320 contracts radially away from the inner surface 339 of the
pulley 318, a guide surface 420 at the second end of the carrier
348 (shown at 422) engages a guide surface 424 at a second end of
the opening 409 such that the surfaces exert a force on one
another. The shape of the guide surface 424 is selected so that it
drives the second end 411 of the carrier 348 to rotate radially
inwardly, thereby bringing the end 344 of the wrap spring 320 away
from the inner surface 339 of the pulley 318. In other words the
guide surface 420 at the second end 422 of the carrier 348
cooperates with the guide surface 424 at the second end of the
opening 409 to cause a radially inward rotation of the carrier 348
to bring the end 344 of the wrap spring 320 away from the inner
surface 339 of the pulley 318. Thus these surfaces 420 and 424
assist in causing radial contraction of the wrap spring 320 when
such contraction is desired.
It will be noted that the metallic carrier 348 and its pivotal
arrangement with the retainer 358 may be applicable to other clutch
assemblies. For example, the carrier 348 and the pivoting
arrangement may be used on clutch assemblies where a magnetic
circuit does not pass through the pulley itself, such as the clutch
assembly shown in US patent publication number 2010/0122882 or in
PCT patent publication number WO2012135942A1, both of which are
incorporated herein by reference. The carrier 348 and its pivoting
arrangement may also be incorporated in other clutch assemblies
that do not involve a magnetic circuit at all, such as in a
decoupler, such as the decoupler disclosed in U.S. Pat. No.
7,618,337. It will be understood that a separate retainer that
mounts fixedly to a first clutch member is not necessary. The first
clutch member itself may have an arcuate torque transfer surface
for engaging the carrier 348. Furthermore, the torque transfer
surface on the carrier 348 may be a concave surface instead of a
convex surface and the torque transfer surface on the first clutch
member or retainer may be a convex surface instead of a concave
surface.
Use of a Pulley Sleeve in the Pulley
Another feature of the clutch assembly 314 is that the pulley 318
may be formed from two components, including a pulley rim 410 and a
sleeve 412. The pulley rim 410 may have a relatively complex shape,
such as an S-shape as shown in the cross-sectional view shown in
FIG. 11c, and may be formed from a low carbon steel so that it is
relatively easy to stamp the complex shape and to roll form the
pulley grooves (shown at 414) that engage a belt (not shown). The
sleeve 412 may be formed from a stronger and/or harder and/or
tougher material so as to resist damage from engagement with the
wrap spring 320. The sleeve 412 may have a generally simple
cylindrical shape with a radially inner surface that may be formed
(e.g. machined) to tight tolerances so as to provide consistent,
predictable engagement with the wrap spring 320. The sleeve 412 may
be made from any suitable material such as a 1045 carbon steel and
may be heat treated.
The sleeve 412 and the pulley rim 410 may be connected to each
other in any suitable way, such as by a splined connection, by a
key, by brazing, by welding, by a press-fit, or by a press-fit with
Loctite between the mating surfaces to strengthen the press-fit
joint.
By providing a sleeve 412 that has a simple cross-sectional shape
from a relatively harder material with tight manufacturing
tolerances and a pulley rim 410 that has a complex cross-sectional
shape from a relatively softer material and optionally, relatively
looser manufacturing tolerances, the cost of the pulley 318 is kept
relatively low, while providing a high degree of strength and/or
hardness and/or toughness, and dimensional predictability where
needed (i.e. where the pulley engages the wrap spring 320).
Furthermore, by providing a separate sleeve and a separate pulley
rim 410, a suitable, inexpensive pulley rim 410 may be combined
with a first sleeve 412 that is selected to have the right material
properties (e.g. strength, hardness, toughness) for a first
application without being over engineered for the application so as
to keep costs low, and the same type of pulley rim 410 may be
combined with a different, more expensive but stronger sleeve 412
for a heavier duty application.
The inner surface 339 of the pulley 318 is, as shown, the inner
surface of the sleeve 412. In order to improve the performance of
the wrap spring 320 against this surface 339, grease or the like
may be provided on the inner surface 339. In order to assist in
keeping the grease in the region of surface 339 during rotation of
the pulley 318, one end of the sleeve 412 may have an inwardly
extending lip 412a, and a separate lip member 413 may be press-fit
or permanently connected in any other suitable way to the sleeve
412. This separate lip member 413 may be referred to as a grease
dam.
Slip Ring
Optionally a slip ring 428 (FIG. 11c) may be provided to assist the
wrap spring 320 to contract radially away from the pulley 318 upon
deenergization of the electromagnet 369. The slip ring 428 is a
ring that is rotatably mounted to the retainer 358. The slip ring
428 may be made from any suitable material (e.g. a polymeric
material) that permits rotational slippage with the retainer 358.
The slip ring 428 may be provided in addition to or alternatively
to providing the guide surfaces 420 and 424 on the carrier 348 and
the retainer opening 409. The slip ring 428 is a ring that is
rotatably mounted on the retainer 358. When the electromagnet 369
is deenergized, the coils of the wrap spring 348 proximate the
second end 342 begin to contract radially away from the inner
surface 339 of the pulley 318. As the coils 344 contract, the
second end 342 of the wrap spring 320 rotates relative to the first
end 340. Without the slip ring 428, as these initial coils 344
contract and engage the retainer 358, they may grip the retainer
358 so tightly that they resist further rotation and thereby
prevent the rest of the coils 344 of the wrap spring 320 from
contracting fully. As a result, some of the coils 344 may continue
to engage the pulley 318 thereby generating an unwanted drag force
on the first clutch member 316 in addition to potentially
continuing to drive the pulley 318 when it is intended for the
pulley 318 to be disengaged. By providing the slip ring 428, as the
coils 344 proximate the second end 342 of the wrap spring 320
contract they grip the slip ring 428, however, these coils 344 can
continue to rotate as needed to permit the rest of the coils 344
proximate the first end 340 to contract radially via slippage of
the slip ring 428 rotationally on the retainer 358. As a result,
the wrap spring 320 can contract as desired upon deenergization of
the electromagnet 369.
The slip ring 428 may be similar to that described and shown in
FIG. 23 of PCT patent publication WO2011156917, the contents of
which are incorporated herein by reference.
While the above description constitutes a plurality of embodiments
of the present invention, it will be appreciated that the present
invention is susceptible to further modification and change without
departing from the fair meaning of the accompanying claims.
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